PO API update of DFF

Hello All,
I am trying to use an API (po_headers_pkg_s2) to update a header level DFF (attribute13).  I am not 100% sure this is the correct API or how to use it.  I would appreciate any assistance to get it working.

This is going in my Forms 6i custom Program Units packages to be called out when a user wants to update the field.

PROCEDURE update_po_loading_date IS
 
   l_header_rec po_headers_pkg_s2.x_po_header_id;
 
  BEGIN
     
    dbms_application_info.set_client_info(:xxprk_europe_v.org_id);      
      l_header_rec := po_headers_pkg_s2.query_row(:xxprk_europe_v.po_header_id);