string url = https://xxx.xxxx.com/Services/Login.ashx;
string json = "";
Encoding encoding = Encoding.UTF8;
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
request.Method = "post";
request.Headers.Add("Method", context.Request.Headers["Method"]);
request.Headers.Add("timespan", context.Request.Headers["Timespan"]);
request.Headers.Add("Nonce", context.Request.Headers["Nonce"]);
request.Headers.Add("Token", context.Request.Headers["Token"]);
request.Headers.Add("Account", System.Web.HttpUtility.UrlDecode(context.Request.Headers["Account"]));
request.Headers.Add("Source", context.Request.Headers["Source"]);
request.Headers.Add("Version", context.Request.Headers["Version"]);
request.Headers.Add("Sign", context.Request.Headers["Sign"]);
request.Accept = "text/html, application/xhtml+xml, */*";
request.ContentType = "application/x-www-form-urlencoded";
byte[] buffer = encoding.GetBytes(RequestString.ToString());
request.ContentLength = buffer.Length;
request.GetRequestStream().Write(buffer, 0, buffer.Length);
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
using (StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.UTF8))
{
json = reader.ReadToEnd();
}