Html5 push notification and android application

Hi all,
I’m trying to create Android app based on Firebase Cloud messaging connected with HA html5 push notification.
But I’m not sure how to put it together.
I’m now able to send notifications to android app from firebase console. (using startup application)

Do somebody knows how it works in HA? Is the HA frontend sending a registration somewhere?

ok, I found, how to receive web push notification by fcm, but I need to decrypt it. Sigh