public class CookieAwareWebClient : WebClient
{
  private CookieContainer cookie = new CookieContainer();
  protected override WebRequest GetWebRequest(Uri address)
  {
    WebRequest request = base.GetWebRequest(address);
    if (request is HttpWebRequest)
    {
      (request as HttpWebRequest).CookieContainer = cookie;
    }
    return request;
  }
}

var client = new CookieAwareWebClient();
client.BaseAddress = @"https://hovertree.net/any/base/url/";
var loginData = new NameValueCollection();
loginData.Add("login", "YourLogin");
loginData.Add("password", "YourPassword");
client.UploadValues("login.php", "POST", loginData);
//Now you are logged in and can request pages
string htmlSource = client.DownloadString("index.php");