How to add device for push notification service GetStreamApi

I am using the getstream api and I would like to enable push notification services on both android and ios devices. The documentation just shows various excerpts on various parts of the process and I don't understand why. This is a nuanced topic and detailed step by step process should be the standard. But essentially, I am not sure how to add a device to a client.

I have tried the below (which i'm not sure is the right way to go about it, but the documentation is not clear) but I get an unauthorized token error:

_invokeGetStreamApiFcmToken() async {
    final client = StreamChatCore.of(context).client;

    //listen for fcm token refresh
    FirebaseMessaging.instance.onTokenRefresh.listen((token) {
      client.addDevice(token, PushProvider.firebase);
      print("get stream api token refreshed -- $token");
    });

    print(await FirebaseMessaging.instance.getToken());
    print(await FirebaseMessaging.instance.getAPNSToken());

    // register the device with APN (Apple only)
    await client.addDevice(uid, PushProvider.apn);

    // register the device with Firebase (Android only)
    await client.addDevice(uid, PushProvider.firebase);

    print(await client.getDevices());
  }

In the client.addDevice() method, I am not sure at all what the first parameter, "device id" is meant to be. How do i get this device ID?