Following code worked file until server IP changed. I use domain in URL.
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(@"https://example.com/oauth/token?grant_type=password&username=" + username + "&password=" + password);
            request.Method = "POST";
            request.Headers.Add("Authorization", "Basic ......");
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            Stream dataStream = response.GetResponseStream();
            StreamReader reader = new StreamReader(dataStream);
            string result = reader.ReadToEnd();
            reader.Close();
            dataStream.Close();
Error:
The request was aborted: Could not create SSL/TLS channel.
But postman does it.
 
    