public class ExtendedEntryCell : Entry
{
    public event EventHandler<EventArgs> KeyPressed;

    public void SendKeyPressed(object sender, EventArgs e)
    {
        KeyPressed?.Invoke(sender, e);
    }
}

protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
{
    base.OnElementChanged(e);

    if (Control == null)
        return;

    Control.KeyDown -= Control_KeyDown;
    Control.KeyDown += Control_KeyDown;
}

private void Control_KeyDown(object sender, Windows.UI.Xaml.Input.KeyRoutedEventArgs e)
{
    if (e.Key == Windows.System.VirtualKey.Enter)
    {
        (Element as ExtendedEntryCell).SendKeyPressed(Element, new EventArgs());
    }
}