Exemplo de como criar um barcode scanner com Xamarin.Forms http://julianocustodio.com/scanner

AppDelegate.cs

        public override bool FinishedLaunching(UIApplication app, NSDictionary options)
        {
            global::Xamarin.Forms.Forms.Init();
            LoadApplication(new App());

            //Inicializa o Zxing no iOS
            ZXing.Net.Mobile.Forms.iOS.Platform.Init();

            return base.FinishedLaunching(app, options);
        }