Using a URL
According to Apples article Handling Subscriptions Billing and WWDC 2018 Session 705 the following url can be used to link to the manage subscription page:
https://apps.apple.com/account/subscriptions
Using StoreKit/SwiftUI (iOS 15+)
UIKit: Use StoreKits showManageSubscriptions(in:)
(Documentation).
SwiftUI: Use SwiftUIs modifier manageSubscriptionsSheet(isPresented:)
(Documentation).