Hi neilrogers1 ,
Forgive me if I get this wrong and someone has to correct me. While I work for MYOB I'm from a different area of the business.
If you look at the API documentation for the endpoint:
https://developer.myob.com/api/myob-business-api/v2/purchase/supplierpayment/
You can retrieve a Payment using the formula you suggest, however part of the response is the "URI" for the individual payment.
In your case this would look like:
/Purchase/SupplierPayment/f9116384-6192-49c6-bda9-xxxxxxxxx
You would then need to send the DELETE request directly to that URI endpoint.
There's also a full Postman collection with examples for the AccountRight API you can find here:
https://accountrightapi.myob.cloud/#fb7216b7-0c12-481a-bdd0-9764f4be3f55
Note: You can't really say "Hey, retrieve me the body" in a DELETE command, so that's likely part of why the call is failing.
Hope this helped, if it didn't, please reply asking for extra information and I'll leave it up to the regular API support team/other developers to get back to you.