Subscription is the one of few objects that doesn't work with like to like API field name copying in Payments2Us. However, with a simple flow on the Payment  Txn (status equals Matching  Complete - so it will not impact any  payments and subscription not equal  to null) that can then update the  subscription record with the relevant  field (e.g. Campaign). 

NOTE: Flows and customisations are NOT included as part of our standard support.  For any further details or information, you will require our Premium Support service.