To make it possible to access the Google spreadsheet, it's necessary to provide Hold It! with OAuth credentials for Google's Cloud API. I used the approach documented by Google for creating a service account and downloading a JSON file containing credentials. Briefly:

1. Create a service account in the Google GCP Console
2. From the Service account drop-down list, select New service account.
3. Enter a name into the Service account name field.
4. From the Role drop-down list, select Project > Owner
5. Click Create. A JSON file that contains your key downloads to your computer.