Send message with Google Business Messages using access token in C#

Question

I am trying to send a google business message in c#, but getting 403 Forbidden error. Below is the code that I have used. Cant figure out what is going wrong.

        var token = GetAccessTokenFromJSONKey("D:\\test\\key.json", "https://www.googleapis.com/auth/businessmessages");
        string url = "https://businessmessages.googleapis.com/v1/conversations/111d4745-8f28-4c7e-acd1-111234fd/messages";

        representative rep = new representative();
        rep.avatarImage = "https://developers.google.com/identity/images/g-logo.png";
        rep.displayName = "XYZ";
        rep.representativeType = "HUMAN";
        GooglePostData gd = new GoogleData { text = "TestMessage", messageId = "123456", representative = rep };


        using (var client = new WebClient())
        {
            client.Headers[HttpRequestHeader.ContentType] = "application/json";
            client.Headers.Add("Authorization", "Bearer "+ token);   
          
            var resp = client.UploadString(url, "POST", JsonConvert.SerializeObject(gd));
        }

//AccessToken is obtained from below methods

       using (var stream = new FileStream(jsonKeyFilePath, FileMode.Open, FileAccess.Read))
        {
            return await GoogleCredential
                .FromStream(stream) // Loads key file
                .CreateScoped(scopes) // Gathers scopes requested
                .UnderlyingCredential // Gets the credentials
                .GetAccessTokenForRequestAsync(); // Gets the Access Token
        }