postWithWebClient.cs

string URI = "http://www.site.com";
string myParameters = "param1=value1&param2=value2&param3=value3";

using (WebClient wc = new WebClient())
{
    wc.Encoding = Encoding.UTF8;
    wc.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded";
    string htmlResult = wc.UploadString(URI, myParameters);
}