This is my code.I made a public sandbox account.Geting an error 401 unauthorized.
var httpWebRequest = (HttpWebRequest)WebRequest.Create("https://platform.devtest.ringcentral.com/restapi/oauth/authorize");
httpWebRequest.Method = "GET";
httpWebRequest.Headers.Add("client_id", "DShb1**********2DlM5xoA");
httpWebRequest.Headers.Add("redirect_uri", "http://localhost:505471/");
httpWebRequest.Headers.Add("response_type", "code");
httpWebRequest.Headers.Add("state", "Client state");
httpWebRequest.Headers.Add("prompt", "login");
var httpresponse = httpWebRequest.GetResponse();