Here is an example how to send notification via FCM with a HTTP request
https://riptutorial.com/firebase/example/31662/firebase-cloud-messaging-http-protocol
Your secret key can be found in the Firebase console under the settings tab (either the web api key under "general" or the sender id under "cloud messaging". I am not quite sure about the required key, I am sorry).
If you use your own server there is a firebase-messaging JS SDK that you can use, too.
