How to store fcm token in the users document at the time of create account using Firebase Authentication

Thanks for your reply. I am facing an issue, please help me out in it, the scenario is that I have a FlutterFlow project, using Firebase Authentication; while when I create an account using LoginPage I am also storing the fields in the collection document named as "users"  but I am not able to store the fcm_token generated by Firebase in the users document against the field fmc_token (plz ignore the fieldname mistake).

I tried to create the Custom Function to get the FCM token but the function not compiled or save

Custom Function:

Future<String?> getFCM() async {
  final FirebaseMessaging fcm = FirebaseMessaging.instance;
  String token = await fcm.getToken() ?? '';

  return token;
}

I also tried to created the custom action and include the custom action along with the AuthCreateAction on the Create Account button but not succeed. 

Therefore, I need help how to store the fcm token generated at the time of account creation in the users document against the field 'fmc_token'

Thankyou in advance and looking forward to your reply.