Getting error POSTing to REST Service

Question

I have a REST service in JAVA that I need to call by using POST.

Following is my call from .Net -

string postData = "version=2.1&serviceName=createToken";

string endPoint = https://devservices.caremark.com:450/onesite/oneSiteToken/createToken + "?" + postData;

// create endpoint
var request = (HttpWebRequest)WebRequest.Create(endPoint);
//WebRequest request = WebRequest.Create(endPoint);
request.Method = "POST";
byte[] postDataBytes = Encoding.UTF8.GetBytes(postData);
request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = postDataBytes.Length;
var dataStream = request.GetRequestStream(); //this is where it erring out: NotSupported Exception
dataStream.Write(postDataBytes, 0, postDataBytes.Length);
dataStream.Close();

var response = (HttpWebResponse)request.GetResponse(); //this comes out null

Can someone please tell me what I am doing wrong or suggest me an alternative way to to this. I am really stuck at this  one :(

Thanks!