The sign in with Apple prereqs state that:
you must have an existing app in the App Store that uses Sign in with Apple
If you want to support sign in with apple on a website but do not need an iOS app specifically for that website is possible to have a "dummy" app that isn't released to the app store but will serve to support sign in with Apple?
The Sign in with Apple Usage Guidelines for Websites and Other Platforms state the following:
If Sign in with Apple is enabled in your app on the App Store, we recommend offering Sign in with Apple on any associated websites or apps for other platforms by using Sign in with Apple JS — a system-provided JavaScript API. An app on the App Store is required in order to use this API.
Cheers,
Paris X Pinkney | WWDR | DTS Engineer