Navigating from App.xaml.cs to a page with a back button to mainpage

I have my notification handlers in app.xaml.cs. When a user taps on a notification sent to them, OnNotificationOpened is called. In the handler, I've added:

MainPage = new CustomNavigationPage(new NotificationPage())

This all works well, but I would like a back button to allow the user go back to my main page from the NotificationPage. Right now when the user taps on the notification, they can't navigate anywhere once on the page. They would have to kill and reopen the app.