main.cs

#Esto va en el load
if (VerifyLicense() == false)
            {
                MessageBox.Show("Licencia vencida contacte a ITCO para continuar");
                Environment.Exit(1);

            }
//la funcion

private bool VerifyLicense() {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(@"http://itco.space/activate.php");
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            string license = File.ReadLines(@"C:\Program Files (x86)\ITCO S.A\Factura Electronica\license.txt").First();
            string postData = "sn="+license;
            byte[] data = Encoding.UTF8.GetBytes(postData);
            Stream dataStream = request.GetRequestStream();
            dataStream.Write(data, 0, data.Length);
            dataStream.Close();
            WebResponse response = request.GetResponse();
            dataStream = response.GetResponseStream();
            StreamReader reader = new StreamReader(dataStream);
            // Read the content.  
            string responseFromServer = reader.ReadToEnd();
            reader.Close();
            dataStream.Close();
            response.Close();
            if (responseFromServer == "601")
            {
                return true;
            }
            else {
                return false;
            }
        }
//ONexit reemplazar 
 Environment.Exit(1);