I want to integrate fb sigin in in android i also used firebase database

Question

class MainActivity : AppCompatActivity() {
    private lateinit var database: FirebaseDatabase
    //private lateinit var myRef: DatabaseReference

    // define the auth variable here
    private  lateinit  var auth: FirebaseAuth

    // here we define the variable
    private lateinit var callbackManager: CallbackManager
    private lateinit var loginButton: LoginButton

    @SuppressLint("MissingInflatedId")
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        FirebaseApp.initializeApp(this@MainActivity)

        database = FirebaseDatabase.getInstance("https://facebookauthorization-422d6-default-rtdb.firebaseio.com/")

        auth = Firebase.auth

//        var database: FirebaseDatabase = FirebaseDatabase.getInstance("https://facebookauthorization-422d6-default-rtdb.firebaseio.com/")

        loginButton = findViewById(R.id.login_button)
        callbackManager = CallbackManager.Factory.create()

        loginButton.setReadPermissions("email")

        // here we initalised the variable
        loginButton.registerCallback(callbackManager, object : FacebookCallback<LoginResult> {
            // <here we add data class >           
    }

class MyApplication : Application() {

    override fun onCreate() {
        super.onCreate()

        FirebaseApp.initializeApp(this@MyApplication)
    }
} 

I also used Main application but not know how to use it properly.