Twitter-api – Firebase Auth – Use Microsoft OAuth AccessToken to access Firebase Rest API

I’m currently trying to use a microsoft access token to access my firebase api (and find out the userid within firebase-auth / google identity platform. Calling https://identitytoolkit.googleapis.com/v1/accounts:signInWithIdp ends in a 400

What have I done:
Get my accesstoken by calling:
https://login.microsoftonline.com/[tenant]/oauth2/v2.0/token
This works well and I get a access_token, refresh_token and id_token in response.

After that I try to authenticate to GIP by using this documented REST Call: https://cloud.google.com/identity-platform/docs/use-rest-api#section-sign-in-with-oauth-credential

Unfortunately there are only examples available for google and twitter. Calling it with this body fails:

{
"postBody": "idToken=[AuthTokenFromMsCall]&providerId=microsoft.com",
    "requestUri":"http://localhost",
    "returnIdpCredential":true,
    "returnSecureToken":true,
    "tenant":"[ms_tenant]" //tried without this as well  
}

but get this response with code 400

{
  "error": {
    "code": 400,
    "message": "INVALID_CREDENTIAL_OR_PROVIDER_ID : Invalid IdP response/credential: http://localhost?idToken=[AccessToken]&providerId=microsoft.com",
    "errors": [
      {
        "message": "INVALID_CREDENTIAL_OR_PROVIDER_ID : Invalid IdP response/credential: http://localhost?[AccessToken]&providerId=microsoft.com",
        "domain": "global",
        "reason": "invalid"
      }
    ]
  }
}

Does anyone know how to configure my body parameters to get a 200 Response