My knowledge may be out of date, but as of ~2012-2015, Apple generally made free-app-with-paid-subscription services choose between one of the following two opt

My knowledge may be out of date, but as of ~2012-2015, Apple generally made free-app-with-paid-subscription services choose between one of the following two options: 1. No sign-up/subscribe/payment buttons in-app, and no links in-app to external payment forms for those purposes, (effectively the app is a convenience for the users you already have) or, 2. Integrate your sign-up/subscription/upgrades available in-app to Apple's systems, and give up a cut for any subscriptions/upgrades that originated via the Apple app. (On this side, you're effectively paying Apple commission for converted leads generated through their app as an advertising channel.) They included this in part of their review process, and they did not give much leeway. Even linking to your service's website from in-app was risky, if your website was plastered with sign-up/subscribe/upgrade buttons.