MainActivity.kt

fun signInWithCustomToken(customToken: String) {
        customToken?.let {
            auth.signInWithCustomToken(it)
                .addOnCompleteListener(this) { task ->
                    if (task.isSuccessful) {
                        // Sign in success, update UI with the signed-in user's information
                        val user = auth.currentUser
                    } else {
                        // If sign in fails, display a message to the user.
                        Toast.makeText(
                            baseContext, "Authentication failed.",
                            Toast.LENGTH_SHORT
                        ).show()
                    }
                }
        }
    }