You can typically purchase a custom URL during the signup process for various Google platforms:
During the initial setup for a Google Workspace account, you will be asked if your business already has a domain. If you select "No, I need one," Google will guide you through purchasing an available domain from Squarespace. google urls buy
If you previously had a domain registered with Google, here is what you need to know about the current status: Use a custom domain for your site - Google Help You can typically purchase a custom URL during