Depending on what type of web service goodfon.su has, the must simple authentication is that the WebClient gets a cookie container in return after login.
The cookie container has to be attached to every web request you are making to goodfon.su, now goodfon.su knows that you are logged on.
Take a look at the
CookieContainer[
^] class
...
CookieContainer _sessionCookie = new CookieContainer();
...
WebClient webRequest = new WebClient()
webRequest.CookieContainer = _sessionCookie;
webRequest.UpdateDataValue(...)
WebClient webDownload = new WebClient();
webDownload .CookieContainer = _sessionCookie;
webDownload.DownloadFile(...);
...