Notification hub to specific device

I am following this tutorial to test out notification hub for android and it works
https://docs.microsoft.com/en-us/azure/notification-hubs/xamarin-notification-hubs-push-notifications-android-gcm

but I need to finish the user registration process and get the userid so I can use the userid as tag to register on Azure Notification hub. So instead of doing the registration right away in MainAcitivty.cs, I just save the token in database.

so instead of having this

public override void OnNewToken(string token)
        {
            Log.Debug(TAG, "FCM token: " + token);
            SendRegistrationToServer(token);
        }

i have this

public override void OnNewToken(string token)
        {
            Log.Debug(TAG, "FCM token: " + token);
            LocalDb.InsertDeviceToken(token);
        }

when the whole registration process is finished, I have following in an acitivty.cs

 var notificationUtil = new NotificationUtil();
 notificationUtil.SendRegistrationToServer(this.ApplicationContext);

following is the code for NotificationUtil.cs

using System;
using System.Collections.Generic;
using Android.Util;
using PantAppLib.source.dbaccess;
using PantAppLib.source.models;
using WindowsAzure.Messaging;

namespace PantAppAndroid.Utils
{
    public class NotificationUtil
    {
        public void SendRegistrationToServer(Android.Content.Context context)
        {
            try
            {
                // Register with Notification Hubs
                NotificationHub hub = new NotificationHub(Constants.NotificationHubName,
                                            Constants.ListenConnectionString, context);
                var userProfile = LocalDb.GetUserProfile();
                if (userProfile != null)
                {
                    var tags = new List<string>() { userProfile.Id };
                    var token = LocalDb.GetDeviceToken().TokenValue;
                    Registration registration = hub.Register(token, tags.ToArray());
                    var regID = registration.RegistrationId;

                    Log.Debug("MyFirebaseMsgService", $"Successful registration of ID {regID}");

                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }
        }
    }
}

I can only guess that I am using the correct context object here, and when I run the code, i hit on following exception

{Android.OS.NetworkOnMainThreadException: Exception of type 'Android.OS.NetworkOnMainThreadException' was thrown.
  at Java.Interop.JniEnvironment+InstanceMethods.CallObjectMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue* args) [0x0006e] in <42748fcc36b74733af2d9940a8f3cc8e>:0 
  at Java.Interop.JniPeerMembers+JniInstanceMethods.InvokeVirtualObjectMethod (System.String encodedMember, Java.Interop.IJavaPeerable self, Java.Interop.JniArgumentValue* parameters) [0x0002a] in <42748fcc36b74733af2d9940a8f3cc8e>:0 
  at WindowsAzure.Messaging.NotificationHub.Register (System.String pnsHandle, System.String[] tags) [0x00043] in <7ef3a358b177460dacd73e56198bd8f2>:0 
  at PantAppAndroid.Utils.NotificationUtil.SendRegistrationToServer (Android.Content.Context context) [0x00041] in C:\Users\weiha\source\repos\App2\PantAppAndroid\Utils\NotificationUtil.cs:36 
  --- End of managed Android.OS.NetworkOnMainThreadException stack trace ---
android.os.NetworkOnMainThreadException
    at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1513)
    at java.net.Inet6AddressImpl.lookupHostByName(Inet6AddressImpl.java:117)
    at java.net.Inet6AddressImpl.lookupAllHostAddr(Inet6AddressImpl.java:105)
    at java.net.InetAddress.getAllByName(InetAddress.java:1154)
    at com.android.okhttp.Dns$1.lookup(Dns.java:39)
    at com.android.okhttp.internal.http.RouteSelector.resetNextInetSocketAddress(RouteSelector.java:175)
    at com.android.okhttp.internal.http.RouteSelector.nextProxy(RouteSelector.java:141)
    at com.android.okhttp.internal.http.RouteSelector.next(RouteSelector.java:83)
    at com.android.okhttp.internal.http.StreamAllocation.findConnection(StreamAllocation.java:174)
    at com.android.okhttp.internal.http.StreamAllocation.findHealthyConnection(StreamAllocation.java:126)
    at com.android.okhttp.internal.http.StreamAllocation.newStream(StreamAllocation.java:95)
    at com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:281)
    at com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:224)
    at com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:461)
    at com.android.okhttp.internal.huc.HttpURLConnectionImpl.connect(HttpURLConnectionImpl.java:127)
    at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.connect(DelegatingHttpsURLConnection.java:89)
    at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.connect(HttpsURLConnectionImpl.java:26)
    at com.microsoft.windowsazure.messaging.Connection.executeRequest(Connection.java:213)
    at com.microsoft.windowsazure.messaging.Connection.executeRequest(Connection.java:170)
    at com.microsoft.windowsazure.messaging.Connection.executeRequest(Connection.java:130)
    at com.microsoft.windowsazure.messaging.NotificationHub.refreshRegistrationInformation(NotificationHub.java:301)
    at com.microsoft.windowsazure.messaging.NotificationHub.registerInternal(NotificationHub.java:399)
    at com.microsoft.windowsazure.messaging.NotificationHub.register(NotificationHub.java:148)
    at mono.android.view.View_OnClickListenerImplementor.n_onClick(Native Method)
    at mono.android.view.View_OnClickListenerImplementor.onClick(View_OnClickListenerImplementor.java:30)
    at android.view.View.performClick(View.java:6597)
    at android.view.View.performClickInternal(View.java:6574)
    at android.view.View.access$3100(View.java:778)
    at android.view.View$PerformClick.run(View.java:25885)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
}

the follow up tutorial - Send notifications to specific devices is not for xamarin android, I am not sure why you have this kind of content structure where you have 3 tutorials, where the second is built upon the first, but they are using totally different tech. Any help?