Id number has nothing to do with the lookup. Idnumber are use to set relationship between two tables. You search the current table and the record pointer will move to the related child record in the second table base on the relationship. Portals also use relationship to show record from both tables.
I figured it out, it seems I was using the lookup target field not the source field. FM Help says "This must be thefor the relationship upon which the is based, not the Lookup source or target field."