posting data to PHP from C#

HI I have been trying to post data to PHP webservice(3rd party) from my C# code. The PHP webservice says it expects a parameter c (missing parameter c is the error I get). I am using JSON to send the data, but i do not understand how do give the parameter. Would be great if some one could throw light on this.The following is my code:

DropYa d = new DropYa();
List<DropYaUser> d1 = new List<DropYaUser>();
DropYaUser ds = new DropYaUser();
ds.action = "create";
ds.groupid = 10;
ds.name = "Test";
ds.manager_key = "test";
d1.Add(ds);

WebRequest request = WebRequest.Create(" http://dev.dropya.net/api/Group.php");
// Set the Method property of the request to POST.
request.Method = "POST";
// Create POST data and convert it to a byte array.
JavaScriptSerializer ser = new JavaScriptSerializer();//typeof(DropYaUser));

string postData = ser.Serialize(ds);
byte[] byteArray = Encoding.UTF8.GetBytes(postData);
// Set the ContentType property of the WebRequest.
request.ContentType = "application/json";

// Set the ContentLength property of the WebRequest.
request.ContentLength = byteArray.Length;
// Get the request stream.

Stream dataStream = request.GetRequestStream();
// Write the data to the request stream.
dataStream.Write(byteArray, 0, byteArray.Length);
// Close the Stream object.
dataStream.Close();
Console.Write("Wrote");
Console.Read();
// Get the response.
WebResponse response = request.GetResponse();
// Display the status.
Console.WriteLine(((HttpWebResponse)response).StatusDescription);
// Get the stream containing content returned by the server.
Console.Read();

dataStream = response.GetResponseStream();
Console.Read();
// Open the stream using a StreamReader for easy access.
StreamReader reader = new StreamReader(dataStream);
// Read the content.
string responseFromServer = reader.ReadToEnd();
// Display the content.
Console.WriteLine(responseFromServer);
// Clean up the streams.
reader.Close();
dataStream.Close();
Console.Read();
response.Close();