Access your iCloud Mail, Calendar, and Contacts in third-party apps
You can access your iCloud Mail, Calendar, and Contacts in supported third-party apps across your devices after you authorize the app using your Apple Account.
Authorize the app
When you set up an account in your supported third-party application, enter your Apple Account email address. When prompted, click Allow to authorize the app to access your data and return to the third-party app. If you make changes to an email address, alias, or your account full name in iCloud Mail, you see these changes after the next time you authorize the app.
If an app doesn't support this, you can sign in using an app-specific password instead.
If you previously used an app-specific password, after you authorize the app to access your Apple Account, revoke your app-specific password.
Revoke your authorization
Sign in at account.apple.com.
Choose Sign-In and Security from the side panel, then select Account Data Sharing.
Select "Remove access" for the app that you want to revoke your authorization from.
After you revoke your authorization, the app cannot access your data until you authorize it again.