Issues when adding a meeting registrant via Graph API

Hey,

We have been trying to add a registrant to a meeting using the MS Graph API. We have managed to authenticate and get the meeting ID, but we cannot add the registrant. 

The meeting was created in Teams as a Webinar.

The error message in the API is very uninformative as well. If we make an obvious mistake, like omit a required field, we get a reasonable error message. But as soon as we put together what looks like a correct request, we just get the below message. 

Can anybody see any obvious issues with our request? The request details are below:

METHOD: POST

URL:

https://graph.microsoft.com/beta/users/659fc6aa-615f-4cfc-8646-e8e4c7ad8c67/onlineMeetings/MSo2NTlmYzZhYS02MTVmLTRjZmMtODY0Ni1lOGU0YzdhZDhjNjcqMCoqMTk6bWVldGluZ19PR0prWTJGa1pqRXRaV1ZsTkMwME1XTXlMVGc0TldNdE9UQXhPVFppWVRjNE1UUmhAdGhyZWFkLnYy/registration/registrants

BODY:

{
  "@odata.type": "#microsoft.graph.meetingRegistrant",
  "firstName": "firstname",
  "lastName": "lastname",
  "email": "email address removed for privacy reasons"
}

HEADERS:

Authorization: Bearer {TOKEN}

Content-Type: application/json

RESPONSE:

{
    "error": {
        "code": "BadRequest",
        "message": "Bad Request",
        "innerError": {
            "date": "2023-02-28T16:31:53",
            "request-id": "7767a8a2-770b-458d-9492-ab46a16f6c9b",
            "client-request-id": "7767a8a2-770b-458d-9492-ab46a16f6c9b"
        }
    }
}

Many thanks!