The JSON key gets downloaded while creating a service account. Your organization should have a G Suite account for this.
Please login to the Google Developers Console using the G Suite admin credential. Click on ‘Create Project’ and create a new one by giving a Project Name. From the navigation menu on the left pane, select API & Services > Credentials > Create credentials >Service account key > New service account.
Kindly provide the following details: Service account name: Provide a suitable name for the service account.
Service account ID: Provide a suitable ID for the service account.
Role: From the drop-down list select Service Accounts > Service Account Admin
Select the Key type as JSON and click Create. A JSON key will be downloaded.
Please find the detailed description on the same using the help link. Keep posting!