Creating a safari app extension and the “beforeSearch” event issue

I'm trying to implement an extension in a modern way: https://developer.apple.com/documentation/safariservices/safari_app_extensions. The idea is to improve Safari search by using keys. For instance, if you start your search with "a" it will use Amazon.com as a searching source.

Previously it was possible to use:

safari.application.addEventListener('beforeSearch', handlerFunc, false);

to react on a search try.

How to implement in a modern way?