Each service has at least one technical domain linked. You can use them to access the service, whether it's an application or a database. Hostman provides you with a free technical domain at .hostman.site, but you can also add your own domain to any service.
Open the Domains menu item on the left.
Enter the name of your domain and click Add domain. Then you need to change the IP address (A record) for this domain in your domain registrar customer panel. When you're finished, click Check IP address and the domain will be added to your service.
If you plan to use the new domain as the primary one, click Set as default.