Instead of passing the channels as arguments, I’m passing a credentials.yml like described here: credentials.yml
In there you can define and specify all the details of the channels you want to use, then you can just pass the credentials.yml with rasa_core.run -c <path-to-credentials.yml>. The rest stays the same.
The credentials.yml with this custom channel just looks like I mentioned above:
channels.facebook.FacebookInput:
verify: ...
secret: ...
page-access-token: ...