If you can use the WebClient
class, using basic authentication becomes simple:
var client = new WebClient {Credentials = new NetworkCredential("user_name", "password")};
var response = client.DownloadString("https://telematicoprova.agenziadogane.it/TelematicoServiziDiUtilitaWeb/ServiziDiUtilitaAutServlet?UC=22&SC=1&ST=2");