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");