FCM with token registration in Firebase database

Hello, I am in doubt about using FCM. Hi, I'm trying to add a token to a webserver, but I'm not sure how to do this. : how to reference the token of the app with the user ID that is your main key in the database, how do I know that the email or username of the user that I am putting in the database is referring to a certain ID. For example, on the site I would list all users' emails, selecting one would forward a notification to that user's device.