For any of our field sets, please never use the related object.  E.g  when the ">" symbol appears. What you need to do is create a custom field on the Payment Txn that has  the same API name as that on the contact.

Then make the field publicly accessible. Steps to be followed: https://help.payments2us.com/m/userguide/l/236183-how-to-add-custom-fields-on-a-web-form