You can't link to a UDT as a data source like you are trying to do in your screenshot.

If you are trying to use the UDT for validation, then create the UDF in AR_Customer with Manual Entry as the source, then set the type to multi-line and the length to same length as the UDT's key column and then on the validation tab, you should be able to select your UDT.  

Keep in mind that an SY UDT shares its data across all companies so if you ever want different values for different companies, you need to create the UDT under a module, pick a module that is appropriate based in its intended usage.

Also, although you can't use a UDT as a data source like you are doing in your screenshot, if I recall correctly, you can have other fields in your UDT based on the selected value in the UDF displayed in the target task by adding them to the panel but they are ready only.  

If you describe what you are trying to accomplish with the UDT and UDF, you may get more specific and detailed answers.

Indeed, Data Sources are for data flow, which can't come from a UDT (since there is no business object that flows data from a UDT to other tables... UDT validation, as David describes, is not a data flow).

Setting default UDF values for new customers is done a different way.