How to get reference token for google business api

I am getting a response from OAuth 2.0 Playground when i hit the URL in the browser i am getting the following error,

URL: https://mybusiness.googleapis.com/v4/accounts/123456789/locations/987654321/reviews

{ "error": { "code": 401, "message": "Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.", "status": "UNAUTHENTICATED" } } 

How to set OAuth to my URL?

How to get a reference token and how to view the response in the browser?