Looking at the event description here
This event is triggered when an application is given access to a repository rather than when a repository is added to an org where the app is installed
This might also help clarify https://developer.github.com/apps/differences-between-apps/
> GitHub Apps aren’t subject to organization application policies. A GitHub App only has access to the repositories an organization owner has granted.
> A GitHub App receives a webhook event when an installation is changed or removed. This tells the app creator when they’ve received more or less access to an organization’s resources.
It might be that what you are looking for is the Repository Event or the RepositoryImportEvent, I don’t quite have enough context to give guidance on that.
Hope this helps.