To use firebase notifications package there are two scenarios :

1 - We have only one server key to use for sending notifications
    
    In that case add one property i.e. ServerKey in your {config}.json file in your project like
    
    "FirebaseMessaging" : {
      "ServerKey" : "YOUR SERVER KEY HERE"
    }

   And next step is to add  RegisterFirebaseMessaging(configuration); in your startup.cs file.

   Third and last step is to inject IFirebaseNotificationService in your class where you want to use it i.e. 
   
  private readonly IFirebaseNotificationService firebaseMessagingService;

  public MyClass(IFirebaseNotificationService firebaseMessagingService){
  this.firebaseMessagingService = firebaseMessagingService;
  }

  // use firebaseMessagingService to call methods where required
  
  // if you have device ids and only have one serverkey
  await firebaseMessagingService.SendNotification(deviceIds, "message", "title");

  // if you have device ids and only have one serverkey and want to send to specific topic
  await firebaseMessagingService.SendNotification(deviceIds, "message", "title", "my topic");

2 - We have multiple server keys ( multiple projects to support ) to use for sending notifications

  In that case we don't need update {config}.json file, just add RegisterFirebaseMessaging(configuration); in Startup.cs

   // if you are supporting multiple server keys ( firebase projects ), and have device ids
   await firebaseMessagingService.SendNotification(serverKey, "message", "title", deviceIds);

  // if you are supporting multiple server keys ( firebase projects ), and have a topic
   await firebaseMessagingService.SendNotification(serverKey, "message", "title", "topic");