You will have received an email sharing a calendar with you.
If you don’t have the custody calendar app, get it from the app store using the link in the email.
Once you have the App, tap the “Open Calendar in App’ link.
Custody Calendar app will open, and will show you what access you have been given to this calendar.
Tap ‘Add’ to add it to your app.
A confirmation will appear letting you know that the calendar has been added successfully.