try
{
ServicePointManager.Expect100Continue = true;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
var webClient = new WebClient();
var s = webClient.DownloadString("https://jtlplugins.x-volution.de/api.php?apikey=yS5VS7OiG1ukiIqLzCSYuFCjeF1qSskKOQeCtVxh&do=pruefe_app&cappid=123&chardwareid=DC0D-BFEA-6F79-58DE-21E9-BA3A-B288-C46F&clizenzschluessel=123");
MessageBox.Show("Result" + s);
}
catch(Exception ex)
{
MessageBox.Show(ex.Message);
}