[C#] Hướng dẫn đăng nhập website sử dụng Http Request with Authenticate Username and password

using System;
using System.Net;
/// 
/// A Cookie-aware WebClient that will store authentication cookie information and persist it through subsequent requests. ///

public class CookieAwareWebClient : WebClient {
 //Properties to handle implementing a timeout
 private int? _timeout = null;
 public int? Timeout { get { return _timeout; }
 set { _timeout = value; }
 }
 //A CookieContainer class to house the Cookie once it is contained within one of the Requests
public CookieContainer CookieContainer { get; private set; }
 public CookieCollection ResponseCookies { get; set; } 
//Constructor
 public CookieAwareWebClient() {
 CookieContainer = new CookieContainer();
 this.ResponseCookies = new CookieCollection();
 }
 public CookieAwareWebClient(CookieContainer cookies)
 {
 this.CookieContainer = cookies;
 }
 //Method to handle setting the optional timeout (in milliseconds)
 public void SetTimeout(int timeout) { 
_timeout = timeout;
 } 
//This handles using and storing the Cookie information as well as managing the Request timeout 
protected override WebRequest GetWebRequest(Uri address)
{
 //Handles the CookieContainer var request = (HttpWebRequest)base.GetWebRequest(address); request.CookieContainer = CookieContainer; //Sets the Timeout if it exists if (_timeout.HasValue) { request.Timeout = _timeout.Value; } return request; 
} 
}

 private void button1_Click(object sender, EventArgs e)
        {
            using (var client = new CookieAwareWebClient())
            {
                client.Encoding = Encoding.UTF8;              
                var values = new NameValueCollection { { "username", textBox1.Text }, { "password", textBox2.Text } };
                client.UploadValues(new Uri("https://www.cloudflare.com/a/login"), "POST", values);
                var html = client.DownloadString(@"https://www.cloudflare.com");
                webBrowser1.DocumentText = html;
            }
        }