Payables adding a DFF

Dear ALL

I faced an issue that every time I create an invoice from a specific PO the requester isn't provided automatically we should take his name from the lines and write it

so I got an idea about creating a DFF in the header that will tale automatically the name from the requestor PO

in that case whenever I issue and invoice the requester will be generated automatically for the approval cycle

do you know the steps or a white paper how can I create it and link it ?

Thank you

Regards