Step 1. Register a new identifier.

(i) Log into your Apple Developer Account After logging in click "Certificates, IDs & Profiles"

(ii) Then in the left nav bar click "Identifiers" and then the "+" button to create a new identifier.

(iii) Select service IDs then Continue

Step 2. Add your app Identifier (Client ID)

Add your app description and your app identifier, which will be the client ID. Press continue and then register.

Step 3. Configure your app identifier

Select the identifier you just created and click "Configure" as shown in the green boxes in the images below.

Step 4. Select the primary app ID and add all permitted Domains and Subdomains, and permitted Return URLs

Note that the Domains and Subdomains should not start with https:// or include a trailing /
e.g. only enter subdomain.yoursite.com

Click "Confirm" and then "Done".

Step 5: Add the Sign in with Apple Capability

Within your XCode project open the Signing & Capabilities tab and add Sign in with Apple

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Error Code 1000

Failing to add the Sign in with Apple Capability can result in the following error:
Error Domain=com.apple.AuthenticationServices.AuthorizationError Code=1000 "(null)"