I think it would help to know by what method/screen you are creating checks. What do you mean by “how do I pass this in the payload”? I’m not a programmer (might be why I’m unfamiliar with the terminology).
I do import checks with Import Scenarios and I am able to pay specific bills with check import scenarios, including AP checks that pay multiple Bills. I hope this example (attachment) helps you. Good luck!
Thanks Laura, have now sorted the issue, as part of the API JSON payload I needed to add a sub-array with the bill DocType & ReferenceNbr, this sorted the initial issue and created the payment against the right bill.