This is the "client_secrets.json" file associated with the service account credential in the Google Cloud Console. To create a service account:
- Select your project from the list.
- Navigate to "APIs & Auth" and then "Credentials"
- Click the big red "CREATE NEW CLIENT ID" button
- Select "Service account" and click "Create Client ID".
- A dialog should pop-up to download the private key file for the new service account, save this in a secure location.
- After clicking, "Okay, got it", you should see a yellow-highlighted "Service Account" section; click on the "Download JSON" button just underneath it. This is the file you must upload here.
To regenerate the client secrets file for an existing service account, follow the above instructions until the "CREATE NEW CLIENT ID" step and instead find the Service Account for which you need a new key and click "Download JSON".