Firebase allows us to send notification messages via our own application by making POST
request.
This tutorial, gives to us instructions how to make this request. However, there is Authorization
field in header where I must to place my own Server key.
Where I can get this Server key? There are no instructions for this.
Update (2024): this server key is no longer available as the API that requires has been sunset. See the documentation on migrating to the v1 API to learn more about its replacement API (which has been available since 2017).
Solution: