using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Threading;
using System.Configuration;
using System.Net;
using System.Text.RegularExpressions;
using System.Data.SqlClient;
using System.IO;
using Microsoft.Win32;
using System.Web;

Daha sonra post edicek kodlarımız ise aşağıdaki gibidir ;

xml="<SingleTextSMS>";
            xml += "<UserName>test</UserName>";
            xml += "<PassWord>test</PassWord>";
                   
            //xml post başla

            string webpageContent = string.Empty;

            try
            {
                byte[] byteArray = Encoding.UTF8.GetBytes("data="+xml);

                HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create("url adresi");
                webRequest.Method = "POST";
                webRequest.ContentType = "application/x-www-form-urlencoded";
                webRequest.ContentLength = byteArray.Length;

                using (Stream webpageStream = webRequest.GetRequestStream())
                {
                    webpageStream.Write(byteArray, 0, byteArray.Length);
                }

                using (HttpWebResponse webResponse = (HttpWebResponse)webRequest.GetResponse())
                {
                    using (StreamReader reader = new StreamReader(webResponse.GetResponseStream()))
                    {
                        webpageContent = reader.ReadToEnd();
                    }
                }
            }
            catch (Exception ex)
            {

                webpageContent = ex.Message;
            }


            MessageBox.Show(webpageContent);