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 authorise the app using your Apple Account.
Authorise 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 authorise 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 authorise 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 authorise the app to access your Apple Account, revoke your app-specific password.
Revoke your authorisation
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 authorisation from.
After you revoke your authorisation, the app cannot access your data until you authorise it again.