public Task<Token> AcquireTokenForApplicationAsync()
{
string uri = string.Format("{0}/{1}/oauth2/token", authUrl, tenant_name);
string contentType = "application/x-www-form-urlencoded";
string content = string.Format("grant_type=client_credentials&client_id={0}&client_secret={1}&resource={2}", app_id, HttpUtility.UrlEncode(app_key), HttpUtility.UrlEncode(graphApiUrl));
APIService service = new APIService(uri, contentType);
return service.SendAsync<Token>(content);
}