'microsoft.substrateConnectors.externalGroup' could not be resolved by the model

Hi, am trying to create an external group, but this isn't working.

Reference Docs: https://docs.microsoft.com/en-us/graph/api/externalconnection-post-groups?view=graph-rest-beta&tabs=http
I have created a connection, schema with some properties. I need to create an externalGroup to restrict access to items for certain users.

I've tried this in postman for example,
POST https://graph.microsoft.com/beta/external/connections/now/groups ("now" is my connection id)

Request Body:

{
  "@odata.type": "#microsoft.graph.externalGroup",
  "id": "testgroup",
  "displayName": "Test Group",
  "description": "Test Group"
}

but it doesn't work, I get a 400 Bad Request error with following response Body
Response:

{
  "error": {
    "code": "InvalidRequest",
    "message": "The request is malformed or incorrect.",
    "innerError": {
      "code": "DeserializationError",
      "message": "A type named 'microsoft.substrateConnectors.externalGroup' could not be resolved by the model. When a model is available, each type name must resolve to a valid type.",
      "date": "2021-03-14T18:41:29",
      "request-id": "60af803c-30e3-4bd7-8d38-0e2fb45f0a0b",
      "client-request-id": "60af803c-30e3-4bd7-8d38-0e2fb45f0a0b"
    }
  }
}

Is this the right way to query this?

Similar Issue can be seen for externalGroupMembers as well. https://docs.microsoft.com/en-us/graph/api/externalgroup-post-members?view=graph-rest-beta&tabs=http

AB#8595

Start with the Microsoft Graph beta external groups documentation linked in the issue and reproduce the POST request in Postman against the documented endpoint. Compare the documented externalGroup and externalGroupMembers types with the service response; the investigation is complete when the supported request shape or a confirmed service-side discrepancy is established.