StoreKit on the Server using Vapor?

I have an iOS app on the App Store with auto-renewal subscriptions using StoreKit 2, and I also have a website but without any subscriptions yet. I'm looking for a way to add subscriptions to the website, plus I'm very keen on rewriting the website using Swift and Vapor.

I would want my users to pay once and be able to use the premium service on both the app and the website.

I've looked into the App Store Server API but I can't find anything to process the initial payment.

Is there a way to achieve this using any of Apple's libraries or Rest APIs, or any third party services? What has been your experience to achieve this?