What you need
For a single-device Firebase push notification test, you need a Firebase service account JSON key and the FCM registration token generated by your client app.
1. Get a service account JSON key
In Firebase Console, open Project settings and then Service accounts. Generate a new private key only for a service account you are authorized to use. The file includes the project ID, service account email and RSA private key used to authorize server requests.
2. Get the FCM device token
Use the Firebase Messaging client SDK in your Android, iOS, Flutter, or web app to obtain the current registration token for the app installation you want to test.
3. Send with the tester
Open the FCM Notification Tester, upload or paste the service account JSON, paste the device token, enter a notification title and message, optionally add a JSON data object, and choose Send Notification.
What happens behind the form
The tool signs a service-account assertion, requests a short-lived OAuth 2.0 access token with the Firebase Messaging scope, and sends the message through the FCM HTTP v1 endpoint for the project in the JSON key.
For the canonical API details, see the Firebase HTTP v1 send documentation.