If you have the "Hide My Email" option active in your Apple ID settings, when you sign in to Fade In Access using "Sign in with Apple" and your Apple ID, it will never know your actual Apple ID: Fade In will instead only be provided a unique "xxxxxxxxxx@privaterelay.appleid.com" email that is specifically linked from Fade In Access to your account.
Unfortunately if Fade In support ever needs to look up your account or billing information, it will be unable to do so using your Apple ID, since it has only ever been told an privaterelay.appleid.com email address.
You can see the privaterelay.appleid.com address being used with Fade In Access for your Apple ID in the desktop browser from the main screen of Fade In Access (showing your file listing) or under Settings > Account.
If you need to look it up outside of Fade In Access, you can do so from your Apple ID settings on either your Mac or iOS device under "Sign in with Apple":