Follow the below steps to connect to the Apple API.
Go to Apple Developer Portal  and login to your account. 
Scroll to the section for Membership details . 
Copy the Team ID  from Apple and paste in the Team ID  field in Social Login. 
Under Certificates, Identifiers and Profiles , click Identifiers . 
Click the blue plus icon at the top of the page. 
Choose App IDs  in this first step and App  as the App Type . 
Add a Description  and Bundle ID . It's recommended to use a reverse-domain value (e.g. io.verbb). 
Ensure to check the checkbox for Sign In with Apple . 
Click the Continue  button, then Register . 
Create a new identifier, this time choosing Services IDs . 
Add a Description  and for the Identifier  use the same value for the Bundle ID  with .client (e.g. io.verbb.client). 
Copy this value used for the Identifier  and paste in the Client ID  field in Social Login. 
Click the Continue  button, then Register . 
Go back to edit the new identifier. 
Ensure to check the checkbox for Sign In with Apple  and click the Configure  button.
Pick the associated Primary App ID  you created earlier. 
Enter the Web Domain  for your site. 
In the Return URLs  field, enter the value from the Redirect URI  field in Social Login. 
Click Save  and the Continue  and Register . 
 
 
Click Keys  from the sidebar. 
Click the blue plus icon to register a new key. Give your key a name, and check the Sign In with Apple  checkbox. 
Click the Configure  button and select your primary App ID you created earlier. 
Copy the Key ID  from Apple and paste in the Key File ID  field in Social Login. 
Click the Download  button (it will only be available once). 
Copy this file to config/social-login in your project. 
Select this file in the Key File Path  field in Social Login. 
 
According to the Apple Human Interface Guidelines(opens new window)  , the login button needs to comply with their requirements. As rendering the login buttons are up to you, be sure to consult this guide.