FavFightersViewModel.cs

public class FavFightersViewModel : BindableObject
{
    ObservableCollection<Fighter> _fighters;

    // Added this...
    public Command<Fighter> FavoriteCommand { get; set; }

    public FavFightersViewModel()
    {
        LoadFighters();
        
        // And this!
        FavoriteCommand = new Command<Fighter>((f) =>
        {
            MessagingCenter.Send(f, "Favorited");
        });
    }
    
    // More code here...
}