How to get device token or FCM token without firebase authentication?

I need to use push notification function but did not use firebase as a backend or for authentication.

as my understanding I still need to have firebase blaze plan to use this function, So I plan to do that. But I don't want to use firebase as a authentication because I already have backend that I use in web version. I want to get device token and send it to backend and all push notification will get trigger in my backend. So how can I get device token or FCM token?

Thanks.