using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Configuration;
using System.Data.SqlClient;
using System.Data;
using System.Diagnostics;
using System.Net;
using System.IO;

namespace Sample2
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                //data
                string cookieStr = "51fd9f14fa7561b5";
                string postData = string.Format("userid={0}&password={1}", "guest", "123456");
                byte[] data = Encoding.UTF8.GetBytes(postData);

                // Prepare web request...
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://www.xxx.com");
                request.Method = "POST";
                //request.Referer = "https://www.xxx.com";
                request.ContentType = "application/x-www-form-urlencoded; charset=UTF-8";
                request.UserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36";
                //request.Host = "www.xxx.com";
                request.Headers.Add("Cookie", cookieStr);
                request.ContentLength = data.Length;
                Stream newStream = request.GetRequestStream();

                // Send the data.
                newStream.Write(data, 0, data.Length);
                newStream.Close();

                // Get response
                HttpWebResponse myResponse = (HttpWebResponse)request.GetResponse();
                StreamReader reader = new StreamReader(myResponse.GetResponseStream(), Encoding.UTF8);
                string content = reader.ReadToEnd();
                Console.WriteLine(content);
                Console.ReadLine();
            }
            catch (Exception)
            {
                throw;
            }
        }
    }
}

/// <summary>
/// 調用短信接口發送短信，需配合模板
/// </summary>
/// <param name="userName">接口用戶名</param>
/// <param name="userPwd">接口密碼</param>
/// <param name="mobile">發送手機號碼</param>
/// <param name="content">發送內容</param>
/// <returns> 返回值大於0,發送成功,系統生成的任務id或自定義的任務id</returns>
public static string PostSms(string userName, string userPwd, string mobile, string content)
{
    string srcString = "-999";
    try
    {
        string postString = string.Format("username={0}&password={1}&mobile={2}&content={3}", userName, MD5(userName + MD5(userPwd)), mobile, content);
        byte[] postData = Encoding.UTF8.GetBytes(postString);//編碼，尤其是漢字，事先要看下抓取網頁的編碼方式  
        string url = "http://xxx/smsSend.do";//地址  
        WebClient webClient = new WebClient();
        webClient.Headers.Add("Content-Type", "application/x-www-form-urlencoded");//采取POST方式必須加的header，如果改為GET方式的話就去掉這句話即可  
        byte[] responseData = webClient.UploadData(url, "POST", postData);//得到返回字符流  
        srcString = Encoding.UTF8.GetString(responseData);//解碼
    }
    catch { }
    return srcString;
}

    class Program
    {
        static void Main(string[] args)
        {
            var str = DownloadStringAsync(new Uri("http://www.baidu.com"));
            Console.WriteLine(str.Result);
            Console.ReadLine();
        }

        static async Task<string> DownloadStringAsync(Uri uri)
        {
            //WebClient 不支持超時設定
            //而HttpWebRequst則允許你設置請求頭或者對內容需要更多的控制
            var webClient = new WebClient();
            webClient.Encoding = System.Text.Encoding.UTF8;
            var result = await webClient.DownloadStringTaskAsync(uri);
            return result;
        }
    }

C# 使用HttpWebRequest Post提交數據,攜帶Cookie和相關參數示例