Hi Cheh Lu,
This is possible when you use FMS (Formatted Search). As you Mentioned "I wish to auto populate a value in UDF after I add the outgoing payment."Please Refer the Screen Shot For Your Requirement...
To acheive this automatically you have to follow these steps.
Create a query in Query Manager. For your reference please follow the screen shot..
Now you have to call this FMS query in UDF field in Outgoing Payment Screen.
Open Outgoing payment and put your cursor on User define field and then follow the screen shot.
Hope It wil help you...