In order to do this you need to declare a dependency in your ext app where you refer to the appsource app
List of dependencies for the extension package. For example: "dependencies": [ {"id": "4805fd15-75a5-46a2-952f-39c1c4eab821", "name": "WeatherLibrary", "publisher": "Microsoft", "version": "1.0.0.0"},{}].
Note: For dependencies to the System Application and Base Application, these are no longer listed as explicit dependencies, but captured in the application setting as a reference to the application package. They must have a version number of the Application package. See application below.
Note: The version specified defines the minimum version for the dependency. At runtime and when downloading symbols, the latest version of the dependency satisfying the specified name, publisher and, minimum version will be returned. When runtime is set to 4.0 or earlier, use appId instead of id.
You can read more about the app.json file here
learn.microsoft.com/.../devenv-json-files
