ViewModel

public class LoginViewModel
    {
        public ICommand LoginCommand { get; set; }

        private User _user;

        public User User
        {
            get { return _user; }
            set { _user = value; }
        }

        public LoginViewModel()
        {
            _user = new User();
            LoginCommand = new Command(Login);
        }

        private void Login()
        {
            var masterPage = new MasterDetailPage()
            {
                Master = new MenuPage(),
                Detail = new NavigationPage(new MainPage())
            };

            if (_user.Username == "yordiagustin" && _user.Password == "123456")
                App.Current.MainPage.Navigation.PushModalAsync(masterPage);
            else
                App.Current.MainPage.DisplayAlert("Error", "Datos Incorrectos", "Ok!");
            return;
        }

    }