Using the right service for a particular requirement is the optimum solution. For this reason we offer our dedicated hosting plans for use with dedicated domain registrars. There are many domain registrars available, and which one you choose is your own decision. A short-list of domain registrars might include: hover.com, mythic-beasts.com, namecheap.com, among many, many, others.
The usual scenario is that you simply take your existing domain, which you enter into the domain field on our registration form, and use your registrar's facilities to point to our hosting servers. Naturally we'll assist you with your DNS configuration if you need help with this.
An alternative free option, if you don't have a domain name of your own, is to use a unique subdomain of rfi.net. In this case you would enter subdomain.rfi.net into the domain field on our registration form. This option is only supported with a managed plan and you may want to discuss this with us first.