How to Set Up Sign in with Apple

To implement Sign in with Apple, you need 

• the Capability setting in Xcode
• the Apple Developer configuration

— these two things.

Setting Up in Xcode

Go to Target > Signing & Capability and add a new item, Sign in with Apple.

Setting Up in Apple Developer

Go to https://developer.apple.com/account/.

Click "Identifiers" (English) under Certificates, Identifiers & Profiles.

Select the relevant app from Identifiers.

Find "Sign in with Apple" here and check the box.