ProForma Invoice (BYD)

Adding an extension field from another business object into a ProForma invoice.
“Pro Forma” invoice is a billing document created in the SAP Business By Design system to simulate real invoice values.  This invoice is created prior to the creation of the built in SAP BYD system invoice and it appears to be constructed exactly the same, once this invoice is created it is not treated as the system generated invoice therefore not requiring any posting to Finance Accounting (FI).