tekin7707 wrote:
I have developed a payment plugin(PaymentMethodType.Redirection) 3.90 and it is worked fine.

also; I create a form in PostProcessPayment and post it(POST method necessary). Page goes to the Bank Payment Page and return result.

        public void PostProcessPayment(PostProcessPaymentRequest postProcessPaymentRequest)
            var myForm = "<form action=\"{0}\" method=\"POST\" />" +
        "<input type=\"hidden\" name=\"secure3dsecuritylevel\" value=\"CUSTOM_PAY\" />" +
              "<input type=\"hidden\" name=\"successurl\" value=\"{1}\" />" +
              "<input type=\"hidden\" name=\"errorurl\" value=\"{2}\" />" +
              "<input type=\"hidden\" name=\"secure3dhash\" value=\"{3}\" />" +
                "<script> document.forms[0].submit();</script>";  

but at 4.00 not worked.

        void IPaymentMethod.PostProcessPayment(PostProcessPaymentRequest postProcessPaymentRequest)

                byte[] data = System.Text.Encoding.UTF8.GetBytes(myForm);

                _httpContextAccessor.HttpContext.Response.Body.Write(data, 0, data.Length);

  As a result, it writes myForm to the page as text.
  PaypalStandart is not a good example for me. I need to send form with POST.

  Is there any solution suggestion?

Thank you Tekin, I'm working on iPara integration. It solved my problem in 4.1 version.