This key is needed to authorize sending push notifications from your server to your app users, this key is needed for http v1 firebase apis.
Follow these steps to get the key for your account.
- Go to https://console.firebase.google.com
- Select the project for which you want to create the key
- Select the settings icon for the android app (most probably you might just have one app).
- Click on Service Accounts tab and click on Generate new private key to download the new key.
- keep this file safe in your server.