FMS for Line Quantity

Hi,

I am trying to fetch the total document quantity by adding the Quantity field - RDR1.Quantity on a UDF - ORDR.U_TotalQuantity.

My FMS looks like this :-

SELECT SUM($[$38.Quantity])

I am not able to get the results, i you can guide either by FMS or by SDK i am ok with both.

Regards