Hi Bogomil,
Please use the “Legacy Server Key” and notice that google stated “Firebase has upgraded our server keys to a new version. You may continue to use your Legacy server key, but it is recommended that you upgrade to the newest version.”
In case you haven’t notice how to setup FCM, here is the guide :
However, you can use FCM Credential as well without problem. I tested and worked perfectly using Firebase API.
Sincerely,
Ariv Ontowiryo