How to Post data in C# . NET HttpWebRequest

using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
using System.Net;

namespace ConsoleApplication1
{
    class Class1
    {
        public string Post(string url, string postData)
        {
            string vystup = null;
            try
            {
                //Our postvars
                byte[] buffer = Encoding.ASCII.GetBytes(postData);
                //Initialisation, we use localhost, change if appliable
                HttpWebRequest WebReq = (HttpWebRequest)WebRequest.Create(url);
                //Our method is post, otherwise the buffer (postvars) would be useless
                WebReq.Method = "POST";
                //We use form contentType, for the postvars.
                WebReq.ContentType = "application/x-www-form-urlencoded";
                //The length of the buffer (postvars) is used as contentlength.
                WebReq.ContentLength = buffer.Length;
                //We open a stream for writing the postvars
                Stream PostData = WebReq.GetRequestStream();
                //Now we write, and afterwards, we close. Closing is always important!
                PostData.Write(buffer, 0, buffer.Length);
                PostData.Close();
                //Get the response handle, we have no true response yet!
                HttpWebResponse WebResp = (HttpWebResponse)WebReq.GetResponse();
                //Let's show some information about the response
                Console.WriteLine(WebResp.StatusCode);
                Console.WriteLine(WebResp.Server);

                //Now, we read the response (the string), and output it.
                Stream Answer = WebResp.GetResponseStream();
                StreamReader _Answer = new StreamReader(Answer);
                vystup = _Answer.ReadToEnd();

                //Congratulations, you just requested your first POST page, you
                //can now start logging into most login forms, with your application
                //Or other examples.
            }
            catch (Exception ex)
            {
                Console.Write(ex.Message);
            }
            return vystup.Trim() + "\n";

        }
    }
}

---

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Class1 cls1 = new Class1();
            string sResponse = cls1.Post(@"http://test.webjet.com.au/webjettsatrial/unauth.aspx", @"PFPageID=FlightFinderTab&PFLayoutID=Layout&__EVENTTARGET=&__EVENTARGUMENT=&__LASTFOCUS=&__VIEWSTATE=%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%2BrUIjKsA31wzEsFFOrcfag%3D%3D&__PREVIOUSPAGE=oyjS_uJrr0APVQWbsJUmfEUfPFMytN5uT70VAkQkkzA1&TSAServerName=&TSALastClientIP=&mail=Enter+your+email+address&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24pnlTripType%24TripType=Return&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlDeparting=SYD&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlDestinationFinder_city=London+Area+Airports&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlDestinationFinder_cityCode=&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlTravelClass=ECONOMY&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlDepartureDate_day=10&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlDepartureDate_month=2010%2F09&ctlDepartureDate_selectedYear=2010&ctlDepartureDate_selectedMonth=9&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlReturnDate_day=20&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlReturnDate_month=2010%2F09&ctlReturnDate_selectedYear=2010&ctlReturnDate_selectedMonth=9&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlAirline1=0&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24ctlAirline2Hidden=0&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24uscNumberOfPassengers%24ctlAdults=1&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24uscNumberOfPassengers%24ctlChildren=0&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24uscNumberOfPassengers%24ctlInfants=0&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24SearchTodayTime=18&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlPickupCity_city=&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlPickupCity_cityCode=&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlPickupDate_day=3&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlPickupDate_month=2010%2F08&ctlPickupDate_selectedYear=2010&ctlPickupDate_selectedMonth=8&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlPickupTime=%3F&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlPickupLocation=NotSpecified&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlDropoffDate_day=4&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlDropoffDate_month=2010%2F08&ctlDropoffDate_selectedYear=2010&ctlDropoffDate_selectedMonth=8&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlDropoffTime=%3F&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlDropoffLocation=NotSpecified&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24ctlCarSize=Any&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24cblCarVendors%240=on&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24cblCarVendors%241=on&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24cblCarVendors%242=on&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24cblCarVendors%243=on&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24CarsTab1%24cblCarVendors%244=on&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24HotelDestination%24Countries=AU&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24HotelDestination%24Destinations=451&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24HotelDestination%24Locations=&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24HotelDestination%24CascadingDropDown1_ClientState=AU%3A%3A%3AAustralia&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24HotelDestination%24ccdCountryDestination_ClientState=451%3A%3A%3ASydney&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24HotelDestination%24ccdDestinationLocation_ClientState=%3A%3A%3A&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24ctlCheckInDate_day=2&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24ctlCheckInDate_month=2010%2F08&ctlCheckInDate_selectedYear=2010&ctlCheckInDate_selectedMonth=8&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24ctlCheckOutDate_day=3&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24ctlCheckOutDate_month=2010%2F08&ctlCheckOutDate_selectedYear=2010&ctlCheckOutDate_selectedMonth=8&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24ctlNumberOfAdults=1&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24ctlNumberOfChildren=0&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24HotelsTab1%24rblResultsViewMode=List&ContentView%24PageLayout%24FlightFinderTab%24currentView=SEARCH&Question=&QuestionLOH=&hiddenInputToUpdateATBuffer_CommonToolkitScripts=0&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24btnSearch.x=0&ContentView%24PageLayout%24FlightFinderTab%24HomepageTabs%24FlightsTab%24btnSearch.y=0");

            Console.WriteLine(sResponse);
            Console.ReadLine();
        }
    }
}

---

Lưu ý để lấy dữ liệu từ Post dạng string dùng tool: Fiddler Web Debugger(v2.2.9.1) ,
sau đó truyền string đó vào phương thức Post trong Class1 -> public string Post(string url, string postData)