To use your own domain name for the forum, follow these steps:

In "Administration - Domain Name" add the domain name that you have registered.

For example: example.com

In the management of DNS zones of your domain registrar, add "CNAME record" eg:

example.com. IN CNAME forum.mybb.ru

Wait for DNS update. This process can take up to several days.

If you did everything correctly, the status of the domain will automatically change to "Working".