Bad Request when Creating an Event Registration

I am attempting to use this endpoint, https://{{URL}}/api/EventRegistration/_execute, in Postman to create a new event registration.

Here is the body of my request.

{
"$type": "Asi.Soa.Events.DataContracts.EventRegistrationRequest, Asi.Contracts",
"EntityTypeName": "EventRegistration",
"OperationName": "RegisterEvent",
"RegistrationType": {
"$type": "Asi.Soa.Events.DataContracts.EventRegistrationType, Asi.Contracts",
"RegistrationType": 3
},
"EventId": "21CONVENT",
"RegistrationOptionFunctionId": "21CONVENT/RO1",
"FunctionId": "21CONVENT/TEST",
"RegistrantId": "80856",
"RegisteredBy": "80856",
"BillTo": "102",
"Waitlist": false
}

Each time, I get a status of 400 Bad Request. Any thoughts on what might be off here.

Thanks,
Korey