FavFightersView.xaml.cs

public partial class FavFightersView : ContentPage
{
    public FavFightersView()
    {
        InitializeComponent();
        BindingContext = new FavFightersViewModel();

        // Catch the message here and show the alert
        MessagingCenter.Subscribe<Fighter>(this, "Favorited", (f) =>
        {
            DisplayAlert("Added favorite", $"{f.Name} is added to your favorites!", "Thanks");
        });
    }
}