How to get value for Client_id and Client_Secret to call API from Postman

Question

I am trying to call RingCentral API from Postman. This is my first attempt and so I am following what is provided by RingCentral documentation: https://developers.ringcentral.com/guide/messaging/sms/postman

I created a Sandbox account and an API app which uses following authorization:

However, when I call from the Postman, I am getting following error:

{
    "error": "unauthorized_client",
    "errors": [
        {
            "errorCode": "OAU-251",
            "message": "Unauthorized for this grant type"
        }
    ],
    "error_description": "Unauthorized for this grant type"
}

Thank you.