If you want to use a custom domain name, you’ll need to register it first. To register a domain name, you’ll need to purchase it from a domain registrar, with costs starting around $10 per year depending on the domain extension and the registrar.
We’re not a domain registrar, so you’ll need to purchase the domain elsewhere. It doesn’t really matter which provider you choose, as long as you can set the nameservers of your domain, which is almost all of them.
After you’ve registered the domain, you can change the nameservers to point the domain to us, and then add it to your hosting account.