Vendor / Business Partner field

I am trying to add "Vendor / Business Partner" to the receipts section of the Travel Expense Statement. So far I have not been able to add this field to the expense report. I have only been able to find the layout for the header not the rest of the document. SPRO -> Travel Management -> Travel Expenses -> Dialog and Travel Expenses Control -> Form Structure -> Modules of Form Variants -> Form Header Structure -> Specfy Layout. Does anyone know where I can find the rest of the document layout?