3.3 WordPress Notification Setup
From your firebase console, go to the Project Settings > Cloud Messaging Tab. You need to enable the Cloud Messaging API (Legacy) which is disabled now.
Click on the Manage API in Google Cloud Console and go to that page and enable the API.
After that, refresh the cloud messaging tab and you see that it is enabled and has a key. Copy that Server Key.
Now go to your WordPress dashboard > Settings > FCM Push Notification from WP and paste the server key in the field of API Key and save that. That's it.
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