In this example we used Microsoft.ProjectOxford.Vision NuGet package: https://www.nuget.org/packages/Microsoft.ProjectOxford.Vision/
To read more about Microsoft Cognitive Services please refer to the official documentation: https://www.microsoft.com/cognitive-services/en-us/computer-vision-api
Please find uploaded sample on my GitHub: https://github.com/Daniel-Krzyczkowski/XamarinIOS/tree/master/XamariniOS_CognitiveServices
I also attach link to my blog where I presented how to use Cognitive Services with Xamarin Forms application: http://mobileprogrammer.pl