Register user using firebaseDB with notifier

How can I make this but using firebase database. My problem is "no valuestored block" in firebaseDB compared to tinyDB.

Use the dataChanged event ?


Thank you for your response appreciated.

My goal is "tag" is the username of the user and "valuetostore" is the password of the user. Every time the user click REGISTER BUTTON without any input in TextBox1 and PasswordTextBox1 it will notify that "Please input username and password" then if the user successfully Register it will notify "Registration Success"