Use the more recent BAdIs ME_PROCESS_PO_CUST for check and ME_GUI_PO_CUST for customer screen (tab)
Some metafield values are reserved for such customer fields: mmmfd_cust_01, the ME_GUI_PO_CUST provide a method ME_GUI_PO_CUST to attach fields to metafield values.
For this SAP provided some macros
- mmpur_metafield -> link to dynpro field
- mmpur_message_forced -> raise an error message
Use those in method PROCESS_ITEM/HEADER and CHECK of ME_PROCESS_PO_CUST.
Perform some search, there are now many threads and sample at scn on those BAdI.
Regards,
Raymond