Created JWT and trying to use this.

App I am using is the only App in Production for sending SMS.

Using Rest Client with C#

restClient = new RestClient(

                "RC_CLIENT_ID",

                "RC_CLIENT_SECRET",

                "https://platform.ringcentral.com");

var jwt = @"REMOVED FOR SECURITY";

var tokenInfo = await restClient.Authorize(jwt);

Response:

StatusCode: 400, ReasonPhrase: 'Bad Request', Version: 1.1, Content: System.Net.Http.HttpConnectionResponseContent, Headers:

{

  Server: nginx

  Date: Fri, 11 Nov 2022 23:44:07 GMT

  Connection: keep-alive

  Vary: Origin

  Vary: Access-Control-Request-Method

  Vary: Access-Control-Request-Headers

  RCRequestId: bbd807d0-621a-11ed-884a-0050568d7d0d

  Pragma: no-cache

  Cache-Control: no-store

  AceRoutingKey: iad41-c01-ace01-f7db789cc-xzsz9_10.120.155.210

  RoutingKey: SJC01P13

  Content-Type: application/json

  Content-Length: 201

  Content-Language: en

}

Content: {

  "error" : "unauthorized_client",

  "errors" : [ {

    "errorCode" : "OAU-251",

    "message" : "Unauthorized for this grant type"

  } ],

  "error_description" : "Unauthorized for this grant type"

}



Request:

Method: POST, RequestUri: 'https://platform.ringcentral.com/restapi/oauth/token', Version: 1.1, Content: System.Net.Http.FormUrlEncodedContent, Headers:

{

  X-User-Agent: Unknown/0.0.1 RingCentral.Net/5.18.0

  Authorization: Basic REMOVED FOR SECURITY

  traceparent: 00-e7fec9cf088e5a8849d254c03b4eea14-a20c80fc5b02ce2f-00

  Content-Type: application/x-www-form-urlencoded

  Content-Length: 770

}

Content: grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=REMOVED-FOR-SECURITY