How to Select Row level field value as input to FMS

Hi,

I required query in which we select Row level field value as input to FMS.

I have added in row level UDF named Size in Sales Order Document.

Requirement is When we select Item code in sales order document required to show the size in that row level UDF from the Item master.

In item master there is a document level UDF named Size.

i have tried query "Select U_Size From OITM where Itemcode=$[RDR1.Itemcode]" But it not worked.