I don't have the answer but...
If you look at the Bizops end points to can find the XML structure for "SaveData" on the "Payment Add Form".
From your CRM splash page, you'd go to "Endpoints > Business Operations Services > [dbname] > Record Creates" and then find in the list "Revenue :: Payment Add Form" and then click "SOAP".
Even if you don't use Bizops instead, I believe (I could be wrong!) the Appfx data should be structured in an equivalent way.
But as it happens I'm just struggling myself at the moment with adding a payment record in Bizops!