A domain name registry is independent from the hosting services. You can purchase the two services from separate companies.
If you want to connect the two services (domain name & hosting) you must declare the nameservers that we provided you with in the web hosting activation email, through the company that provided you with the domain name (registrar). The nameservers have this form ns1.top.host and ns2.top.host
Note: The nameservers mentioned above are an example. Nameservers vary and have forms like ns10, ns58, ns70, etc.