I just found the solution and yes it was easier than I thought :)

so here is the solution:

 string URI = "http://www.myurl.com/post.php"; string myParameters = "param1=value1&param2=value2&param3=value3"; using (WebClient wc = new WebClient()) { wc.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded"; string HtmlResult = wc.UploadString(URI, myParameters); }

works like a charm :)

There is a built in method called UploadValues that can send HTTP POST (or any kind of HTTP methods) AND handles the construction of request body (concatenating parameters with "&" and escaping characters by url encoding) in proper form data format:

using(WebClient client = new WebClient())
{
    var reqparm = new System.Collections.Specialized.NameValueCollection();
    reqparm.Add("param1", "<any> kinds & of = ? strings");
    reqparm.Add("param2", "escaping is already handled");
    byte[] responsebytes = client.UploadValues("http://localhost", "POST", reqparm);
    string responsebody = Encoding.UTF8.GetString(responsebytes);
}

Using WebClient.UploadString or WebClient.UploadData you can POST data to the server easily. I’ll show an example using UploadData, since UploadString is used in the same manner as DownloadString.

byte[] bret = client.UploadData("http://www.website.com/post.php", "POST",
                System.Text.Encoding.ASCII.GetBytes("field1=value1&field2=value2") );
 
string sret = System.Text.Encoding.ASCII.GetString(bret);

More: http://www.daveamenta.com/2008-05/c-webclient-usage/