CustomNavigationPageRenderer.cs

[assembly:ExportRenderer(typeof(BaseNavigationPage),typeof(BaseNavigationPageRenderer))]
namespace yournamespace
{
    class CustomNavigationPageRenderer : NavigationPageGradientHeaderRenderer
    {
        protected override void OnElementChanged(VisualElementChangedEventArgs e)
        {
            base.OnElementChanged(e);

            if (e.OldElement != null)
            {
                return;
            }

            
        }

        private bool set;

        public override UINavigationBar NavigationBar
        {
            get
            {
                if (set)
                {
                    return base.NavigationBar;
                }

                var nav = base.NavigationBar;

                var item = nav.Items.FirstOrDefault();

                if (item != null)
                {
                    var navPage = (NavigationPage)Element;

                    UIImageView view = new UIImageView(UIImage.FromFile(navPage.Icon.File));
                    view.AddGestureRecognizer(new UITapGestureRecognizer(() =>
                    {
                        vm.IsPresented = true;
                    }));
                    item.SetLeftBarButtonItem(new UIBarButtonItem(view), true);
                    set = true;
                }

                return nav;
            }
        }
    }
}