GetACSToken() method return (400) Bad Request

[alexchen2016]

I test the code from https://github.com/Azure/azure-sdk-for-media-services/blob/dev/samples/MediaServicesRESTSample/RESTAPISamples/RESTAPISamples/Program.cs, I get error An unhandled exception of type 'System.Net.WebException' occurred in System.dll.Additional information: The remote server returned an error: (400) Bad Request. when running the code below:

 HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(accessControlUri);
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            request.KeepAlive = true;
            string token = null;

            //Note: You need to insert your client Id and secret into this string in order for it to work.
            var requestBytes = Encoding.ASCII.GetBytes("grant_type=client_credentials&client_id=" + clientId + "&client_secret=" + HttpUtility.UrlEncode(clientSecret) + "&scope=urn%3a" + scope);
            request.ContentLength = requestBytes.Length;

            // Write the request data to the request stream
            var requestStream = request.GetRequestStream();
            requestStream.Write(requestBytes, 0, requestBytes.Length);
            requestStream.Close();

            // Get the response back from the server
            var response = (HttpWebResponse)request.GetResponse();

Thank you for reviewing my issue. Any help will be welcome.