Backbutton.cs

 BackKeyPress += OnBackKeyPress;
 
   private void OnBackKeyPress(object sender, CancelEventArgs e)
        {
            //Code to disable the back button
            e.Cancel = true;

            //Here you can add your own code and perform any task
            var mm = MessageBox.Show("Do you wish to exit the app", "Exit", MessageBoxButton.OKCancel);
            if (mm == MessageBoxResult.OK)
            {
                Application.Current.Terminate();
            }
        }