This repository contains an implementation of Iyzico Payment System on GrandNode. Iyzico has a dotnet API which does not support .net core yet. Therefore, I had to use unoffical version of the API which is implemented by armut.com (it is a popular website in Turkey.) You can take a look into the armut's repository here.
The plugin first tries to process as regular payment (ProcessPayment). If the card requires 3d secure payment (Error DEBIT_CARDS_REQUIRES_3DS) the plugin redirects the user to 3d secure payment method (PostProcessPayment). Thus, the payment completes as success or failure. On the failure, the plugin adds a order note that only admins can see.
I have added a custom value on the payment form that 3d secure payment method. If the customer selects that checkbox the plugin redirects the user's browser to PaymentIyzicoController.ThreeDPayment Action and shows the html content of iyzico api result. Then PaymentIyzicoController.Handler handle the rest of the payment.
Iyzico has a different approach than regular payment system. It requires order information at the beginning (ProcessPayment) but GrandNode does not supply those information at that method. For that, I will update my local version which I won't going to share any soon but you can do it yourself and open an/a issue/PR to GrandNode repository. You can start with change the method signatures on IPaymentMethod interface.