Changing transaction text for Project Invoice

transactionTxt.setKey2(
_custVendPaymProposalLine.custVendTable().Name);

Then I have set up transaction text for Vendor - Payment, Vendor to use Vendor payment %5 to include the Vendor name in the payment transaction text.

Now, I am looking to do the same for Project invoices.  Currently it is set up with Project invoice %2, but I want to include the project contract and End customer name to the transaction text.  

Does anyone of you have any tips to which class(es) and method(s) to modify to be able to do this?

Best regards

Thor