LoginPage.xaml.cs

public partial class LoginPage : StartupPage
{
    public ICommand NextCommand { get; }

    public LoginPage() : base()
    {
        InitializeComponent();
        NextCommand = new Command(async() =>
        {
            await CompleteAsync(new StartupTaskParameters()
            {
                {"Name" , userNameEntry.Text }
            });
        });
    }
}