I opened a purchase requisition in my system using ME53N, navigated to item level tab 'Contact Person' to see AFNAM field 'Requisitioner'.
F1 help suggests the related structure as MEREQ3328-AFNAM.
So I think F4 needs to be added to structure MEREQ3328 instead.
For comparison, same tab has field 'Crea. Ind.' == MEREQ3328-ESTKZ, and it has a dropdown.
Open structure MEREQ3328 reveals that input help is assigned to ESTKZ, and not to AFNAM.