Hi Diogo,
Although the Word Count module is part of the Editor I haven't made any API to interact with it. This is for an upcoming version that I have planned but not have started yet. If you want to get a head start you can read the documentation, clone this component and then make the needed changes yourself.
Documentation: Word and character count - CKEditor 5 Documentation
The magic happens in the Reacting to updates section.
Greetings,
Vincent