Before you can start integrating (or even testing) Sign in with Apple you need a paid membership to the Apple Developer Program. Sign in with Apple is one of the restricted services which is not available for free with just an Apple ID (Membership Details).
Setup
01. Register an App ID
If you don’t have one yet, create a new one at https://developer.apple.com/account/resources/identifiers/list/bundleId following these steps:
Description and Bundle ID, and select the Sign In with Apple capabilityIn case you already have an existing App ID that you want to use with Sign in with Apple:
If you have change your app’s capabilities, you need to fetch the updated provisioning profiles (for example via Xcode) to use the new capabilities.
02. Create a Service ID
Next go to https://developer.apple.com/account/resources/identifiers/list/serviceId and follow these steps: