The remote server returned an error: (403) Forbidden in C#

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C#4.0

webscraping

The remote server returned an error: (403) Forbidden in C#

HttpWebRequest request = WebRequest.Create("https://xxxxxx/extranet/inventory/rate/") as HttpWebRequest;
request.Method = "POST";
request.Accept = "*/*";
request.ContentType = "application/x-www-form-urlencoded; charset=UTF-8";
request.ContentLength = postdata.Length;
request.CookieContainer = cookie;
request.UseDefaultCredentials = true;
//request.UserAgent = "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.86 Safari/537.36";
request.Referer = "https://xxxx.com/extranet/";
request.Proxy.Credentials = System.Net.CredentialCache.DefaultCredentials;
request.Headers[HttpRequestHeader.Cookie] = sessioncookie;

//request.Headers.Add(HttpRequestHeader.Cookie, sessioncookie);

byte[] byteArray = Encoding.UTF8.GetBytes(postdata);
request.ContentLength = byteArray.Length;

Stream dataStream = request.GetRequestStream();
dataStream.Write(byteArray, 0, byteArray.Length);
dataStream.Close();
StreamReader response = new StreamReader(request.GetResponse().GetResponseStream()); =====>got error
var res = response.ReadToEnd();