To install Moodle on a subdomain, you will need to have a subdomain and if you don't have it yet, check this article on how to create it.
Once the subdomain is ready, you can proceed with the installation. You can do it by accessing the Auto Installer menu in Websites section at your account's hPanel.
Only fill your subdomain's name (like info.demokb.xyz as shown in the example above) in the URL section of the configuration window, press the Install button and you'll be good to go! :)