Custom domains and free subdomains

Free subdomain: in the panel go to More > Subdomain and create one. It points at your server automatically.
Buying or transferring a domain: search at https://billing.atbphosting.com/domains . See Buying, transferring and managing domains for the steps. Domains are sold for one year and are not renewed through us. A domain name cannot be changed after it is bought; a different spelling is a new domain.
Domains are not refundable once registered.
After buying, open a ticket with your server address (IP:port) so staff can set it up. If you bought a domain elsewhere, point it with an A record to your server IP and an SRV record (_minecraft._tcp) with your port, or move its nameservers to Cloudflare first.
Bedrock players still need to enter the port unless your server uses the default Bedrock port.