This is an easy way to make a JSON POST request to a remote service using C#:

HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
request.Method = "POST";
request.ContentType = "application/json; charset=utf-8";
DataContractJsonSerializer ser = new DataContractJsonSerializer(data.GetType());
MemoryStream ms = new MemoryStream();
ser.WriteObject(ms, data);
String json = Encoding.UTF8.GetString(ms.ToArray());
StreamWriter writer = new StreamWriter(request.GetRequestStream());
writer.Write(json);
writer.Close();

Notice that in line 4 we create an instance of DataContractJsonSerializer (Assembly: System.ServiceModel.Web) and initialize it with the type of object we are serializing as JSON to send to the service. In lines 5 and 6 the serializer writes the object into a Stream. Line 7 transforms the Stream into an UTF-8 String (as the content-type) and finally in lines 8 to 10 we send the data to the service.