Get the google-services.json file and API key

1. Go on the Firebase console: https://console.firebase.google.com/
2. Create or use an existing project
   

   

3. On the Project Overview, add an application
   

   

4. Enter your package name (get it on the setting page of the notifications in the plugin)
   

   

5. Download the file and ignore the other steps
   

   

6. Click on the Settings icon (top left) > Project settings 
   
7. Go on the Cloud Messaging tab
   

   

8. Copy paste the Server key