public static void SetHeaderValue(WebHeaderCollection header, string name, string value)
{
var property = typeof(WebHeaderCollection).GetProperty("InnerCollection",BindingFlags.Instance | BindingFlags.NonPublic);
if (property != null)
{
var collection = property.GetValue(header, null) as NameValueCollection;
collection[name] = value;
}
}

HttpWebRequest rq = (HttpWebRequest)WebRequest.Create("Url");
rq.Method = "GET";
SetHeaderValue(rq.Headers, "Host", "127.0.0.1");
SetHeaderValue(rq.Headers, "Connection", "keep-alive");
SetHeaderValue(rq.Headers, "Accept", "*/*");
SetHeaderValue(rq.Headers, "X-Requested-With", "XMLHttpRequest");
SetHeaderValue(rq.Headers, "User-Agent", "...");
SetHeaderValue(rq.Headers, "Referer", "http://127.0.0.1/index.php?m=Index&a=indexs");
SetHeaderValue(rq.Headers, "Accept-Encoding", "gzip, deflate");
SetHeaderValue(rq.Headers, "Accept-Language", "1.5");
SetHeaderValue(rq.Headers, "Cookie", "This is Cookie");
 
HttpWebResponse resp = (HttpWebResponse)rq.GetResponse();
 
using (Stream stream = resp.GetResponseStream())
{
StreamReader reader =new StreamReader(stream,Encoding.Default);
string responseString = reader.ReadToEnd();
MessageBox.Show(responseString);
}