Sales Order UDF not Populating (using Business Object)

SOLVED

I'm trying to populate the UserCode into a UDF on the Sales Order Header.I have created UDF_USERCODE String (3) and on the Data Sources tab selected the Business Object: SO_SalesOrder_bus, Data Source: UserCreatedKey, Column: UserCode.
UDF_USERCODE is always empty whether I create a new SO or edit an existing SO.
Please let me know what I have configured wrong?

Thanks,
Danny