Formatted Search syntax in UDF

Good day

I have created siple formatted search to link a Sales Order number to the Incomming Payment header

SELECT T0.[DocNum] FROM ORDR T0 WHERE T0.[CardCode] = $[$5.0.0] and T0.[DocStatus] = 'O'

When I run the Formatted Search to the Reference field it works perfectly, but if i run it in a UDF on the header I get no result. Can you please tel me how the syntax must change to work in a UDF?

Erika Boshoff