とはどういう意味ですか?
HTTP Post メソッドの ContentType = "application/x-www-form-urlencoded"..??
私のコードは
Uri url = new Uri(" http://blah/blah/blah...json");
HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
webRequest.Method = "POST";
webRequest.ContentType = "application/x-www-form-urlencoded";