It is no longer possible to use a server key to send messages through Firebase Cloud Messaging.

You will nowadays needs to use Google Application Default Credentials, a service account JSON file, or a short-lived OAuth2 access token derived from a service account to authorize requests. See the Firebase documentation for full details on how to do this: https://firebase.google.com/docs/cloud-messaging/auth-server

Also see these topics in the Firebase FAQ:

• Which FCM APIs were deprecated on June 20, 2023, and what should I do if I am using those APIs?
• Can new projects enable the legacy Cloud Messaging API?