The order process on the website does not allow you to sign up for a web hosting plan without a domain name. However, you can add a web hosting plan to a domain name in your JuHub account Control Panel, whether the domain is registered with JuHub or simply declared on the DNS zone as being registered with a different service provider, or non-existent.
This domain name will be used if it exists, otherwise it will simply be a reference linked to the hosting plan. Even if your domain name and web hosting plan expire, you will be able to renew one or the other independently.