Formatted Search header Info

Hi Experts,

I require the SO Number to appear on the target document (A/R Invoice) via a formatted search.

SELECT  a.DocNum

FROM ORDR a

WHERE a.DocEntry = $[INV1.BaseEntry]

I can get the information to display on the Invoice lines, but having difficulty getting the same onto the header.

Any help would be highly appreciated.

Regards,

Kiran