Jtdcftul

i would like to add this text Kundennummer: {customer specific number that I change manually}

Perfect would be right under:
newcommand{invoiceReference}{2019textunderscore01textunderscore14textunderscore1} % Rechnungsnummer (z.B. 20150122-4)

So that the customers number appears in the red rectangle:

documentclass[a4paper]{scrlttr2}



usepackage[top=2cm, bottom=1cm, left=2cm, right=2cm]{geometry}

usepackage{graphicx}

usepackage{lmodern}

usepackage[utf8]{inputenc}

usepackage[T1]{fontenc}

usepackage[ngerman]{babel}

usepackage{color}

usepackage[hidelinks]{hyperref}



usepackage[german]{invoice}





%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

makeatletter

defUnitDay            {Preis/Tag}% <========================================



newcommand{FeeDay@Title}{% <==========================================

    \

    noindenttextbf{Activity}&&UnitDay&Count&Amount (BC)\

    hline%

}%



newcommand{FeeDay}[3]{% Yields state 3, Print Fee Item <==============

    %

    % #1 Contents

    % #2 Fee per Day

    % #3 Unit Count

    %

    ifcaseFlag % 0: Invoice not started yet

        error@message{MissingOpening}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 1: Start invoice

        %

        error@message{MissingProject}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 2: Start project, print title

        %

        FeeDay@Title%

        Fee@Line{#1}{#2}{#3}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 3: Print remuneration item

        %

        Fee@Line{#1}{#2}{#3}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 4: Print subtotal remuneration

        %

        warning@message{FeeSTExists}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 5: Expense item

        %

        error@message{FeeBeforeExpense}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 6: Print subtotal expenses

        %

        error@message{FeeBeforeExpense}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 7: Print subtotal project

        %

        error@message{ProjectCompletedNoFee}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 8: Print total, close invoice

        %

        error@message{InvoiceCompletedNoFee}%

        %

        %

    else error@message{InternalError}%

    fi%

}%

makeatother





renewcommand{familydefault}{sfdefault}



% include meta data

include{_data} % <============== delete in this file the following data!

% ################## invoice DATA ##################

newcommand{invoiceDate}{today} % Datum der rechnungsstellung, change `today' to a different  date, if you do not    want to use todays date

newcommand{payDate}{DD.MM.YYYY} % Datum der Zahlungsfrist

newcommand{invoiceReference}{2019textunderscore01textunderscore14textunderscore1} % Rechnungsnummer (z.B.  20150122-4)

newcommand{invoiceSalutation}{Sehr geehrte Damen und Herren,} % die Anrede

newcommand{invoiceText}{folgende Positionen werden in Rechnung gestellt:} % Rechnungstext

newcommand{invoiceEnclosures}{} % encl{} einfügen

newcommand{invoiceClosing}{Mit freundlichen Grüßen}

% ################## invoice DATA ##################



% ################## Customer DATA ##################

newcommand{customerCompany}{Firma XYZ} %ggf. Firma

newcommand{customerName}{Max Mustermann} % Name

newcommand{customerStreet}{Robert-Koch-Str. 12} % Straße

newcommand{customerZIP}{12345} % Postleitzahl

newcommand{customerCity}{Musterstadt} % Ort

% ################## Customer DATA ##################



setkomavar{fromname}{senderName}

setkomavar{fromaddress}{senderStreet \ senderZIP  senderCity}

setkomavar{place}{senderCity}

setkomavar{date}{invoiceDate}

setkomavar{fromphone}{senderTelephone}

setkomavar{frommobilephone}{senderMobilephone}

setkomavar{fromemail}{href{mailto:senderEmail}{nolinkurl{senderEmail}}}

setkomavar{subject}{Rechnung}

setkomavar{location}{

  \[baselineskip] \[baselineskip]

  flushright small {vspace{1cm}Rechnungsnummer: 

  invoiceReference}

}



setkomavar{firsthead}{hfill

   parbox[t][headheight][t]{7.2cm}{

   footnotesize

   raggedright

   flushright

   color[gray]{.3}%

    begin{tabular}{rl}

         Anschrift & usekomavar{fromname}\

         & senderStreet\

         & senderZIP   senderCity \

         \

         %Telefon  Telefon:  ` senderTelephone \ %<===================  remove the `%' and go into `_data.tex' and     add an phone number

         Mobil  & usekomavar{frommobilephone} \

         E-Mail  & usekomavar{fromemail} \

         %Webseite & {normalfontttfamily  senderWeb } \ %<===================  remove the `%' and go into   `_data.tex' and add an URL

         \

         Steuernummer & taxID \

         \

         Institut & accountBankName \

         IBAN & accountIBAN \

         BIC & accountBIC

    end{tabular}                   

   }%

}





begin{document}

    begin{letter}{customerCompany \ customerName \ 

    customerStreet \ customerZIP  customerCity}



            opening{invoiceSalutation}

            invoiceText

            begin{invoice}{Euro}{0}



ProjectTitle{Tagesleistungen} % <====================================== Tagesleistungen

FeeDay{Freelance Arbeit}{300.00}{15}



ProjectTitle{Projekttitel} % <========================================= Stundenleistungen

Fee{Überstunden}{30.00}{2}









%Auslagen % <=================================================Auslagen

%EBC{Hotel, 12 Nächte} {2400.00} % EBCi{Hotel, 12 Nächte} {2400.00}

%STExpenses



% Rabatt

Discount{Rabatt} {0} % <========================================== Rabatt

            end{invoice}

    ps Bitte überweisen Sie den Rechnungsbetrag auf das unten aufgeführte Konto. 

    ps Kleinunternehmerregelung: Im ausgewiesenen Betrag ist gemäß § 19 UStG keine Umsatzsteuer enthalten.



        closing{invoiceClosing}

        invoiceEnclosures







    end{letter}

end{document}  

The trick here is to change the content of command location to

setkomavar{location}{%

  \[baselineskip] \[baselineskip]

  flushrightsmall Rechnungsnummer: invoiceReference\[1pt]

  Kundennummer: customerNumber

}

I added the line Kundennummer: customerNumber which means you have to add a new command customerNumber to the datas for your customer(s). It is better to use a command that writing it each time you write an invoice new. It gives you less errors.

But now you have to define this command like:

 newcommand{customerNumber}{1234567} % Kundennummer

Best solution would be to define two new commands like

 definvoicenr{Rechnungsnummer:}%

 defcustomernr{Kundennummer:}%

to make your invoice more language independent (off course you need then to define this words for the other languages too). Then you can use

setkomavar{location}{%

  \[baselineskip] \[baselineskip]

  flushrightsmall invoicenr invoiceReference\[1pt]

  customernr customerNumber

}

To avoid the splitting of Rechnungsnummer: and current number you can use shorter Names like

 definvoicenr{Rechnungsnr.:}%

 defcustomernr{Kundennr.:}%

With the complete code (based on your last question with the described changings above)

documentclass[a4paper]{scrlttr2}



usepackage[top=2cm, bottom=1cm, left=2cm, right=2cm]{geometry}

usepackage{graphicx}

usepackage{lmodern}

usepackage[utf8]{inputenc}

usepackage[T1]{fontenc}

usepackage[ngerman]{babel}

usepackage{color}

usepackage[hidelinks]{hyperref}



usepackage[german]{invoice}





%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

makeatletter

defUnitDay            {Preis/Tag}% <========================================



newcommand{FeeDay@Title}{% <==========================================

    \

    noindenttextbf{Activity}&&UnitDay&Count&Amount (BC)\

    hline%

}%



newcommand{FeeDay}[3]{% Yields state 3, Print Fee Item <==============

    %

    % #1 Contents

    % #2 Fee per Day

    % #3 Unit Count

    %

    ifcaseFlag % 0: Invoice not started yet

        error@message{MissingOpening}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 1: Start invoice

        %

        error@message{MissingProject}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 2: Start project, print title

        %

        FeeDay@Title%

        Fee@Line{#1}{#2}{#3}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 3: Print remuneration item

        %

        Fee@Line{#1}{#2}{#3}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 4: Print subtotal remuneration

        %

        warning@message{FeeSTExists}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 5: Expense item

        %

        error@message{FeeBeforeExpense}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 6: Print subtotal expenses

        %

        error@message{FeeBeforeExpense}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 7: Print subtotal project

        %

        error@message{ProjectCompletedNoFee}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 8: Print total, close invoice

        %

        error@message{InvoiceCompletedNoFee}%

        %

        %

    else error@message{InternalError}%

    fi%

}%

makeatother





renewcommand{familydefault}{sfdefault}



% include meta data

include{_data} % <============== delete in this file the following data!

% ################## invoice DATA ##################

newcommand{invoiceDate}{DD.MM.YYYY} % Datum der rechnungsstellung

newcommand{payDate}{DD.MM.YYYY} % Datum der Zahlungsfrist

newcommand{invoiceReference}{20150122-4} % Rechnungsnummer (z.B. 20150122-4) YYYYMMDD-1

newcommand{invoiceSalutation}{Sehr geehrte Damen und Herren,} % die Anrede

newcommand{invoiceText}{Für die von mir erbrachte Leistung erhalten sie

hiermit die Rechnung. Bitte zahlen Sie den unten aufgeführten Gesamtbetrag

unter Angabe der Rechnungsnummer (invoiceReference) bis

zum payDate  auf das angegebene Konto ein.} % Rechnungstext

newcommand{invoiceEnclosures}{} % encl{} einfügen

newcommand{invoiceClosing}{Mit freundlichen Grüßen}

% ################## invoice DATA ##################



% ################## Customer DATA ##################

newcommand{customerCompany}{Firma XYZ} %ggf. Firma

newcommand{customerName}{Max Mustermann} % Name

newcommand{customerStreet}{Robert-Koch-Str. 12} % Straße

newcommand{customerZIP}{12345} % Postleitzahl

newcommand{customerCity}{Musterstadt} % Ort

newcommand{customerNumber}{1234567} % Kundennummer <==================

% ################## Customer DATA ##################



setkomavar{fromname}{senderName}

setkomavar{fromaddress}{senderStreet \ senderZIP  senderCity}

setkomavar{place}{senderCity}

setkomavar{date}{invoiceDate}

setkomavar{fromphone}{senderTelephone}

setkomavar{frommobilephone}{senderMobilephone}

setkomavar{fromemail}{href{mailto:senderEmail}{nolinkurl{senderEmail}}}

setkomavar{subject}{Rechnung}



definvoicenr{Rechnungsnr.:}% <========================================

defcustomernr{Kundennr.:}%



setkomavar{location}{%

      \[baselineskip] \[baselineskip]

      flushrightsmall invoicenr invoiceReference\[1pt] % <=========

      customernr customerNumber

    }





setkomavar{firsthead}{hfill

   parbox[t][headheight][t]{7cm}{% 

   footnotesize

   raggedright

   flushright

   color[gray]{.3}%

    begin{tabular}{rl}

         Anschrift & usekomavar{fromname}\

         & senderStreet\

         & senderZIP   senderCity \

         \

         %Telefon  Telefon:  ` senderTelephone \

         Mobil  & usekomavar{frommobilephone} \

         E-Mail  & usekomavar{fromemail} \

         Webseite & {normalfontttfamily  senderWeb } \

         \

         Steuernummer & taxID \

         \

         Institut & accountBankName \

         IBAN & accountIBAN \

         BIC & accountBIC

    end{tabular}                   

   }%

}





begin{document}

    begin{letter}{customerCompany \ customerName \ 

    customerStreet \ customerZIP  customerCity}



            includegraphics[width=0.15textwidth]{logo.png}            

            opening{invoiceSalutation}

            invoiceText

            begin{invoice}{Euro}{0}

ProjectTitle{Projekttitel} % 

Fee{Musterdienstleistung 1}{30.00}{4}

Fee{Musterdienstleistung 2}{36.00}{7}

ProjectTitle{Tagesleistungen} % 

FeeDay{Musterdienstleistung 3}{360.00}{3}



% Auslagen

%EBC{Hotel, 12 Nächte} {2400.00} % EBCi{Hotel, 12 Nächte} {2400.00}

%STExpenses



% Rabatt

Discount{Rabatt} {20} % 

            end{invoice}

        ps Im ausgewiesenen Betrag ist gemäß § 19 UStG keine Umsatzsteuer 

        enthalten.



        closing{invoiceClosing}

        invoiceEnclosures







    end{letter}

end{document}

you get the result:

If you want Kundennummer and rechnungsnummer and the relevant numbers to be one below the other simply change location to

setkomavar{location}{%

      \[baselineskip] \[baselineskip]

      flushrightsmall invoicenr\[1pt] invoiceReference\[1pt] % <=========

      customernr\[1pt] customerNumber

    }

with the result:

Please see that you can use

newcommand{invoiceReference}{2019_01_14_01}

to generate an invoice reference number 2019_01_14_01 but I would not recommend that. Better use 2019-01-14-01 ...

You could just use a tabular environment with two lines for the reference:

documentclass[a4paper]{scrlttr2}



usepackage[top=2cm, bottom=1cm, left=2cm, right=2cm]{geometry}

usepackage{graphicx}

usepackage{lmodern}

usepackage[utf8]{inputenc}

usepackage[T1]{fontenc}

usepackage[ngerman]{babel}

usepackage{color}

usepackage[hidelinks]{hyperref}



usepackage[german]{invoice}



newcommandsenderName{Foo Bar}

newcommandsenderStreet{Foo Street}

newcommandsenderZIP{12345}

newcommandsenderCity{Bar City}

newcommandsenderMobilephone{0123 3456789}

newcommandsenderEmail{foo@bar.de}

newcommandtaxID{4654TE132E}

newcommandaccountBankName{MyBank}

newcommandaccountIBAN{DE12 3456 7890 1234 5678 90}

newcommandaccountBIC{ABCDEFG0HIJ}



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

makeatletter

defUnitDay            {Preis/Tag}% <========================================



newcommand{FeeDay@Title}{% <==========================================

    \

    noindenttextbf{Activity}&&UnitDay&Count&Amount (BC)\

    hline%

}%



newcommand{FeeDay}[3]{% Yields state 3, Print Fee Item <==============

    %

    % #1 Contents

    % #2 Fee per Day

    % #3 Unit Count

    %

    ifcaseFlag % 0: Invoice not started yet

        error@message{MissingOpening}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 1: Start invoice

        %

        error@message{MissingProject}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 2: Start project, print title

        %

        FeeDay@Title%

        Fee@Line{#1}{#2}{#3}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 3: Print remuneration item

        %

        Fee@Line{#1}{#2}{#3}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 4: Print subtotal remuneration

        %

        warning@message{FeeSTExists}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 5: Expense item

        %

        error@message{FeeBeforeExpense}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 6: Print subtotal expenses

        %

        error@message{FeeBeforeExpense}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 7: Print subtotal project

        %

        error@message{ProjectCompletedNoFee}%

        %

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    or % 8: Print total, close invoice

        %

        error@message{InvoiceCompletedNoFee}%

        %

        %

    else error@message{InternalError}%

    fi%

}%

makeatother





renewcommand{familydefault}{sfdefault}



% include meta data

include{_data} % <============== delete in this file the following data!

% ################## invoice DATA ##################

newcommand{invoiceDate}{today} % Datum der rechnungsstellung, change `today' to a different  date, if you do not    want to use todays date

newcommand{payDate}{DD.MM.YYYY} % Datum der Zahlungsfrist

newcommand{invoiceReference}

  {%

    begin{tabular}{@{}r@{}}

      2019textunderscore01textunderscore14textunderscore1\

      MyCustomer123%

    end{tabular}%

  } % Rechnungsnummer (z.B.  20150122-4)

newcommand{invoiceSalutation}{Sehr geehrte Damen und Herren,} % die Anrede

newcommand{invoiceText}{folgende Positionen werden in Rechnung gestellt:} % Rechnungstext

newcommand{invoiceEnclosures}{} % encl{} einfügen

newcommand{invoiceClosing}{Mit freundlichen Grüßen}

% ################## invoice DATA ##################



% ################## Customer DATA ##################

newcommand{customerCompany}{Firma XYZ} %ggf. Firma

newcommand{customerName}{Max Mustermann} % Name

newcommand{customerStreet}{Robert-Koch-Str. 12} % Straße

newcommand{customerZIP}{12345} % Postleitzahl

newcommand{customerCity}{Musterstadt} % Ort

% ################## Customer DATA ##################



setkomavar{fromname}{senderName}

setkomavar{fromaddress}{senderStreet \ senderZIP  senderCity}

setkomavar{place}{senderCity}

setkomavar{date}{invoiceDate}

setkomavar{fromphone}{senderTelephone}

setkomavar{frommobilephone}{senderMobilephone}

setkomavar{fromemail}{href{mailto:senderEmail}{nolinkurl{senderEmail}}}

setkomavar{subject}{Rechnung}

setkomavar{location}{

  \[baselineskip] \[baselineskip]

  flushright small {vspace{1cm}Rechnungsnummer: 

  invoiceReference}

}



setkomavar{firsthead}{hfill

   parbox[t][headheight][t]{7.2cm}{

   footnotesize

   raggedright

   flushright

   color[gray]{.3}%

    begin{tabular}{rl}

         Anschrift & usekomavar{fromname}\

         & senderStreet\

         & senderZIP   senderCity \

         \

         %Telefon  Telefon:  ` senderTelephone \ %<===================  remove the `%' and go into `_data.tex' and     add an phone number

         Mobil  & usekomavar{frommobilephone} \

         E-Mail  & usekomavar{fromemail} \

         %Webseite & {normalfontttfamily  senderWeb } \ %<===================  remove the `%' and go into   `_data.tex' and add an URL

         \

         Steuernummer & taxID \

         \

         Institut & accountBankName \

         IBAN & accountIBAN \

         BIC & accountBIC

    end{tabular}                   

   }%

}





begin{document}

    begin{letter}{customerCompany \ customerName \ 

    customerStreet \ customerZIP  customerCity}



            opening{invoiceSalutation}

            invoiceText

            begin{invoice}{Euro}{0}



ProjectTitle{Tagesleistungen} % <====================================== Tagesleistungen

FeeDay{Freelance Arbeit}{300.00}{15}



ProjectTitle{Projekttitel} % <========================================= Stundenleistungen

Fee{Überstunden}{30.00}{2}









%Auslagen % <=================================================Auslagen

%EBC{Hotel, 12 Nächte} {2400.00} % EBCi{Hotel, 12 Nächte} {2400.00}

%STExpenses



% Rabatt

Discount{Rabatt} {0} % <========================================== Rabatt

            end{invoice}

    ps Bitte überweisen Sie den Rechnungsbetrag auf das unten aufgeführte Konto. 

    ps Kleinunternehmerregelung: Im ausgewiesenen Betrag ist gemäß § 19 UStG keine Umsatzsteuer enthalten.



        closing{invoiceClosing}

        invoiceEnclosures







    end{letter}

end{document}