Copy google-service.json file to main project directory (the same directory of config.xml) for Android.
Look google-service.json if api-key is not empty. If so, download it again.
ionic run android to run app on Android
ionic prepare ios and run project in XCode - copy file into main directory Tree and follow that steps
Go to your project in https://console.firebase.google.com and select Notifications from menu -> *New message -> select your app, add message and click send