It's possible that you initially signed up with Apple and opted to Hide my email when prompted to share your email address. In such cases, Apple generates a private relay email address for you and it will look similar to abcd1234@privaterelay.appleid.com.
On your iPhone or iPad:
Open your device's settings.
Click on your Name - Apple ID, iCloud, Media and Purchases.
Tap on Password and Security.
Choose Sign in with Apple or Apps Using Apple ID.
Select the Hallow app.
You will see your Relay Email under Hide My Email - This App Received.
On your desktop or laptop:
Sign in to appleid.apple.com.
Go to the Sign in & Security tab.
Click Sign in with Apple.
Click Hallow.
You will see your Relay Email under Hide My Email.
We’re always here to help when you need it. You can reach out through this purple chat window on the bottom-right corner or e-mail us at support@hallow.app
God bless 🕊️