How to POST over HTTP from a Windows Form using C#?

I think you will need to use a web service for this kind of operation. Have a look on HttpPostClientProtocol class in MSDN.
good luck

In the documentation index for VS.NET, type "ClientPOST sample".  This is a sample on how to use .NET classes to build a POST.  There are other samples for building GETs and other web requests.

I have included the source in case you don't have direct access to MSDN.

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******************** Copy from here **********************
using System;
using System.Net;
using System.IO;
using System.Text;
using System.Web;

class ClientPOST {
    public static void Main(string[] args) {

        if (args.Length < 1) {
            showusage();
        } else {
            if (args.Length < 2 ) {
                getPage(args[0], "s1=food&s2=bart");
            } else {
                getPage(args[0], args[1]);
            }
        }

        Console.WriteLine();
        Console.WriteLine("Press any key to continue...");
        Console.ReadLine();

        return;
    }

    public static void showusage() {
        Console.WriteLine("Attempts to POST into to a URL");
        Console.WriteLine();
        Console.WriteLine("Usage:");
        Console.WriteLine("ClientPOST URL [postdata]");
        Console.WriteLine();
        Console.WriteLine("Examples:");
        Console.WriteLine("ClientPOST http://www.microsoft.com s1=food&s2=bart");
    }

    public static void getPage(String url, String payload) {
        WebResponse result = null;

        try {

            WebRequest req = WebRequest.Create(url);
            req.Method = "POST";
            req.ContentType = "application/x-www-form-urlencoded";
        StringBuilder UrlEncoded = new StringBuilder();
        Char[] reserved = {'?', '=', '&'};
        byte[] SomeBytes = null;

        if (payload != null) {
        int i=0, j;
        while(i<payload.Length){
            j=payload.IndexOfAny(reserved, i);
            if (j==-1){
            UrlEncoded.Append(HttpUtility.UrlEncode(payload.Substring(i, payload.Length-i)));
            break;
            }
            UrlEncoded.Append(HttpUtility.UrlEncode(payload.Substring(i, j-i)));
            UrlEncoded.Append(payload.Substring(j,1));
            i = j+1;
        }
        SomeBytes = Encoding.UTF8.GetBytes(UrlEncoded.ToString());
                req.ContentLength = SomeBytes.Length;
                Stream newStream = req.GetRequestStream();
        newStream.Write(SomeBytes, 0, SomeBytes.Length);
                newStream.Close();
            } else {
                req.ContentLength = 0;
            }


            result = req.GetResponse();
            Stream ReceiveStream = result.GetResponseStream();
            Encoding encode = System.Text.Encoding.GetEncoding("utf-8");
            StreamReader sr = new StreamReader( ReceiveStream, encode );
            Console.WriteLine("\r\nResponse stream received");
            Char[] read = new Char[256];
            int count = sr.Read( read, 0, 256 );
            Console.WriteLine("HTML...\r\n");
            while (count > 0) {
                String str = new String(read, 0, count);
                Console.Write(str);
                count = sr.Read(read, 0, 256);
            }
            Console.WriteLine("");
        } catch(Exception e) {
            Console.WriteLine( e.ToString());
            Console.WriteLine("\r\nThe request URI could not be found or was malformed");
        } finally {
            if ( result != null ) {
                result.Close();
            }
        }
    }
}