Материал из .Net Framework эксперт
NameValueCollection.Add
using System;
using System.Collections.Specialized;
using System.Net;
using System.Text;
public class UploadValuesTest
{
public static void Main(string[] argv)
{
WebClient wc = new WebClient();
string uri = "http://localhost/testform.aspx";
NameValueCollection nvc = new NameValueCollection();
nvc.Add("lastname", "Blum");
nvc.Add("firstname", "Rich");
byte[] response = wc.UploadValues(uri, nvc);
Console.WriteLine(Encoding.ASCII.GetString(response));
}
}