You don't need to have a registered domain name when signup hosting account. You can put an un-registered domain in hosting signup form.
Hosingbay will provide a temporary URL to access your account. Temporary URL will start with Hostingbay server IP address, For example, http://22.214.171.124/~example/
Once you have a registered domain, it will automatically link to your account. If the domain that you registered is different from the one that you use to sign up the account, you can email email@example.com to modify account domain.
You can sign up the hosting account first although it is better to know that the domain name is available so you must check the availability or else you may have complications when you try to connect your domain name with the hosting. Therefore it is advised to register the domain first and then sign up for hosting.