This repo contains the sample shown during the App Extensibility Session during Build 2016
After cloning the sample, open the solution in Visual Studio. Make sure you're on the Insider version of the Windows Anniversary Update(Build 14295) and you have the associated SDK. This sample is a complete App Extension Host and also shows cropping via LaunchURIForResultsAsync, and includes the Cortana App Service.
To use actual app extensions, clone the following projects and deploy them from Visual Studio. They should automatically appear in the Extensions tab of the main application.