https://[api_key]:[password_key]@pay.payorange-pay.com/index.php/api/v1/widget/transactions

curl https://pay.payorange-pay.com/index.php/api/v1/widget/transactions \
                  -u [API_KEY]:[PASSWORD_KEY] \
                  -d reference_id=400 \
                  -d amount=400 \
                  -d currency=USD \
                  -d client_ip=192.168.0.1 \
                  -d email=client@domain.ltd \
                  -d description=Example credit card charge \
                  -d pay_method=card

      using (var client = new WebClient())
            {
                NetworkCredential myCreds = new NetworkCredential("API_KEY", "PASSWORD_KEY");
                client.Credentials = myCreds;
                byte[] bret = client.UploadData("https://pay.payorange-pay.com/index.php/api/v1/widget/transactions/", "POST",
               System.Text.Encoding.ASCII.GetBytes("reference_id=hello&amount=world;currency=RUB;client_ip=192.168.0.1;email=client@domain.ru;description=value2;pay_method=card"));

                string sret = System.Text.Encoding.ASCII.GetString(bret);
            }

  var request = WebRequest.Create("https://pay.payorange-pay.com/index.php/api/v1/widget/transactions?reference_id=hello&amount=world&currency=RUB&client_ip=192.168.0.1&email=client@domain.ru&;description=value2&pay_method=card");
            string userName = "API_KEY";
            string userPassword = "PASSWORD_KEY";
            string authInfo = userName + ":" + userPassword;
            authInfo = Convert.ToBase64String(Encoding.Default.GetBytes(authInfo));
            request.Headers["Authorization"] = "Basic " + authInfo;
            var response = request.GetResponse();