Google uses service accounts to allow back-end services to talk to one another. Navigate to the service account section within the Google Cloud Console and click on "Create Service account".
On this screen you'll want to name your account something helpful, in this case, we're naming it "Commonality" since that's the service that will be using this account.
You'll also want to grant your service user the ability to run BigQuery jobs. We need this permission to enable running queries against your database, this enables things like KPIs and OKRs powered by your BigQuery data.
After your service account is created click on it within the list of service accounts. Click on the "Keys" tab for the account and create a new key for the account.
Download the key as JSON and copy the result into the "Key for service account" field within Commonality.
You'll also want to specify the region that your data is located in if it's not in the US. By default this is "US" but you'll want to switch this to "EU" if that's where your data is stored.