PO Header DFF default not working

Summary:

We have created a PO Header DFF with Table Type value set. This Table Type value set is fetching the supplier site of the PO.

Now we have a requirement to default the value of the supplier site name on the Header DFF (on PO).

We have written the SQL on DFF to default it also.

"Select substr(Vendor_site_code,1,150) from poz_supplier_sites_V
where
Vendor_ID = :{PARAMETER.SupplierId}
and
vendor_site_id = :{PARAMETER.SupplierSiteId}"

But when we are creating the PO, this defaulting is not working and the DFF is blank.

Can anyone please let us know what could be the reason and how to achieve this requirement?