How to search for "Pay on Receipt" invoice by PO number?

If we create a "Pay on receipt" invoice, the PO number is visible on the invoice lines but not the invoice header.  This means that we cannot search for any of these invoices by PO number.

Is there a way to get the PO number onto the header if it is on the lines, or a way to search by PO number if it is on the lines?

Thanks,

Heidi