This can be accomplished by using the BBDISTRIBUTIONS_fld_DESCRIPTION field in the CAPInvoice object as shown in the code snippet below:

       With .Distribution.Add
          .Fields(BBDISTRIBUTIONS_fld_DEBITACCTNUM) = "01-1000-00"
          .Fields(BBDISTRIBUTIONS_fld_CREDITACCTNUM) = "01-2000-00"
          .Fields(BBDISTRIBUTIONS_fld_AMOUNT) = "7"
          .Fields(BBDISTRIBUTIONS_fld_DESCRIPTION) = "Test Distribution"
       End With

For a more complete example, please refer to the article below:
How to write code to add an invoice

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