Firebase Cloud Messaging HTTP v1 API

In the Google Cloud Console service accounts page, create or select a service account in the Google Cloud project linked to your Firebase project, generate a JSON key if you need one, and use it to mint an OAuth 2.0 access token. Firebase's FCM guide says to authorize send requests with a short-lived access token in the Authorization: Bearer <access_token> header, typically using the https://www.googleapis.com/auth/firebase.messaging scope (or https://www.googleapis.com/auth/cloud-platform). See Authorize with a Service Account from a Different Project and the general send via HTTP v1 API guide.