DroidRenderer.cs

[assembly: Xamarin.Forms.ExportRenderer(typeof(LandscapePage), typeof(LandscapePageRenderer))]
namespace Mobile.Core.Droid.Renderers
{
    public class LandscapePageRenderer : PageRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<Page> e)
        {
            base.OnElementChanged(e);

            ((FormsAppCompatActivity)Context).RequestedOrientation = ScreenOrientation.Landscape;
        }

        protected override void Dispose(bool disposing)
        {
            base.Dispose(disposing);

            ((FormsAppCompatActivity)Context).RequestedOrientation = ScreenOrientation.Sensor;
        }
    }
}

iosRenderer.cs

[assembly: ExportRenderer(typeof(LandscapePage), typeof(LandscapePageRenderer))]
namespace Mobile.Core.iOS.Renderers
{
    public class LandscapePageRenderer : PageRenderer
    {
        public override bool ShouldAutorotate()
        {
            return false;
        }

        public override UIInterfaceOrientationMask GetSupportedInterfaceOrientations()
        {
            return UIInterfaceOrientationMask.Landscape;
        }

        public override UIInterfaceOrientation PreferredInterfaceOrientationForPresentation()
        {
            return UIInterfaceOrientation.LandscapeLeft;
        }
    }
}

LandscapePage.cs

    public class LandscapePage : ContentPage
    {
    }