custom payment

Did you have a look at https://docs.kentico.com/k12/e-commerce-features/customizing-on-line-stores/payment-related-customizing/creating-a-custom-payment-gateway

And although old there is a custom integration video for Kentico 6 at https://devnet.kentico.com/articles/developing-custom-payment-gateway Note that the API changed a lot, but it should help you get the concept...