Example of how to implement TopTabbedPage in iOS applications using Xamarin.Forms - http://julianocustodio.com/toptabbedpage-ios

MainTabbedPage.xaml.cs

using Xamarin.Forms.Xaml;
using Naxam.Controls.Forms;

namespace TopTabbedPageDemo
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class MainTabbedPage : TopTabbedPage
    {
          public MainTabbedPage()
          {
              InitializeComponent();
          }
    }
}