1
Create or select a Firebase project
In the Firebase console, create or open the project associated with your Android app.
2
Enable Firebase Cloud Messaging API v1
Open Project settings → Cloud Messaging. If Firebase Cloud Messaging API (V1) is disabled, open it in Google Cloud Console and enable it.
3
Generate a service-account JSON key
Go to Project settings → Service Accounts and generate a new private key. Save the downloaded JSON file securely.
4
Add the key in OneSignal
In OneSignal, go to Settings → Push & In-App → Push Platforms → Google Android (FCM). Activate the platform, then upload the JSON file under Service Account JSON.
5
Save the configuration
If prompted, select Firebase Cloud Messaging API (V1), then save and continue.