How to send message from laravel to user's phone number using firebase?

I'm making a website using Firebase and I want to send otp to the user when they register their new account.

This is my function to sent otp to user by their phone number.

public function sendOTP($phone)
  {
    $code = $this->generateOTPCode();

    // save to auth otp table
    $auth_otp = AuthOTP::create([
      'phone' => $phone,
      'code' => $code,
      'status' => BaseConstants::STATUS_INACTIVE
    ]);

    if ($auth_otp) {
      // Send otp to phone number via sms service
      $messaging = app('firebase.messaging');
      $registrationToken = // how can I get the device token of user? 
      $message = CloudMessage::withTarget('token', $registrationToken)
        ->withNotification(Notification::create('Mã xác thực OTP', "Mã xác thực của bạn là: $code"))
        ->withData(['otp_code' => $code]);

      $messaging->sendMulticast([$message], $registrationToken);

      //session(['otp' => $otp]);

      $messaging->send($message);
    }
    return $auth_otp;
  }

I tried to use Firebase cloud message, but I don't know how to get device token. So, how can I get the device token from user?

The device token is generated on the user's device, for instance, in a mobile app.
The code to generate the token should be similar to the one bellow.

FirebaseMessaging.getInstance().token.addOnCompleteListener(OnCompleteListener { task ->
    if (!task.isSuccessful) {
        Log.w(TAG, "Fetching FCM registration token failed", task.exception)
        return@OnCompleteListener
    }

    // Get new FCM registration token
    val token = task.result

    // Log and toast
    val msg = getString(R.string.msg_token_fmt, token)
    Log.d(TAG, msg)
    Toast.makeText(baseContext, msg, Toast.LENGTH_SHORT).show()
})

You can check here for a detailed explanation.
Normally after generation, you send the token to your backend for later use.