Use Forms Personalization on the Purchase Order Screen to Default a Value

I would like to use a forms personalization to default a value on the purchase order screen.  I want to take a value entered on a DFF on the requisition header and populate it on the PO form.  Below is what I am putting in the value field on the forms personalization screen.

=select attribute5 from po_requisition_headers_all

where segment1 = :PO_DISTRIBUTIONS.REQ_HEADER_REFERENCE_NUM

and org_id =

:PO_HEADERS.ORG_ID

When I validate, it returns the correct value, but it does not insert it into the related field when I go back to the PO screen.  What am I doing wrong?