Sending push notification to specific user using Facebook sid

Hi there,

I build Xamarin.Forms App using this tutorial: https://docs.microsoft.com/en-us/azure/app-service-mobile/app-service-mobile-xamarin-forms-get-started
and I choose .NET back end.
I implement push notification for android and ios using this: https://docs.microsoft.com/en-us/azure/app-service-mobile/app-service-mobile-xamarin-forms-get-started-push
and also implement authentication using this: https://docs.microsoft.com/en-us/azure/app-service-mobile/app-service-mobile-xamarin-forms-get-started-users
while choosing Facebook as my authentication provider.

My app is running and working great! but now I trying to add push notification to a specific user and I have few questions:

1. Is it possible to use the Facebook sid as the unique id and decide that if I want to send push to a specific user, I will do this using the Facebook sid?
2. I read that when the user does the authentication through Facebook, his Facebook sid is register automatically has a unique id and I can send to this Facebook id as a TAG and it will arrived to the specific user.
   In this line of code: var result = await hub.SendTemplateNotificationAsync(templateParams);
   that is located in my .NET back end code in the " public async Task PostTodoItem(TodoItem item) " method, I tried to add a TAG after the "templateParams" , I tried to enter the Facebook sid or the Installation id, is this correct?
   If I want to send to a specific user, is that the place that I need to add his "TAG"?
   Am I going the right way? can I choose the TAG to be the unique id and when I want to send push to a specific user, I will send to his TAG?
3. I Wanted to know how the user is register to a specific TAG? if the approach from the second question is correct.

*If you have a written code or different approach , I would really like to hear, I trying to implement the push to specific user for about 2 weeks and still did not succeed.

Thank you very much!