If you use any of the Firebase Authentication features that send emails to users, including email link sign-in, email address verification, etc, you must add an email to your Apple Developer Account. This allows Apple to send emails sent by Firebase Authentication to users who opted to hide their email addresses.
To add an email to the Apple developer account:
Open your Apple developer account.
Click on the Certificates, Identifiers & Profiles card.
Click on more in the left side menu.
Click on Configure button.
Click on the (+) button on the right side of Email Sources.
Enter the email in the Email Addresses section and click Next.
Apple Sign In functionality does not work in Run Mode and can only be tested on a real device or emulator. To deploy and test the authentication, you will need to download the code and run it in your IDE.
Once you downloaded the code you must configure your project in Xcode before running and testing your app on a real device. This includes adding a team to your project and adding the sign-in with apple functionality.
Here's how you configure your project in Xcode:
Open your project in Xcode. If you are using Android Studio, right-click on the ios folder, find Flutter and then click on the Open iOS module in Xcode.
In Xcode, click on Runner (left side menu) and then select the Signing and Capabilities tab.
Under the Signing section, find the Team dropdown and select your team.
Now, click on the + Capability (just below the Signing and Capabilities tab) find the Sign in with Apple, and double click to add.
Configuring project in Xcode
7. Testing Apple sign-in
Apple Sign-In can not be tested in Run Mode. Instead, you will need to test it on a real device/emulator.
Open and Run the app from your preferred IDE.
When you see the app running on your device, click on the Apple Login button.