how to call web api in asp.net c# code behind

Question

string json = "{\"clientName\":\"test12\"}";
var httpWebRequest = (HttpWebRequest)WebRequest.Create("https://test/api/ClientID.pdo");
 httpWebRequest.ContentType = "application/json";
 httpWebRequest.Accept = "application/json";
httpWebRequest.Method = "POST";
string credentials = "XYZ:XYZ";
httpWebRequest.Headers.Add("Authorization", "basic" + credentials);
using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
 {
      streamWriter.Write(json);
      streamWriter.Flush();
      streamWriter.Close();
 }
var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();
 using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
 {
    var result = streamReader.ReadToEnd();
 var res = new JavaScriptSerializer().Deserialize<Response>(result);
    int clientID = res.clientID; //return res;
}

I am getting clientID=0.

 

Plz help me.