The Add Website feature allows you to have several websites under the same hosting account. Make sure that you purchase the domain name separately first.
There are two places where you can create an add-on domain:
In hPanel click on Hosting at the top of the page and choose Add Website:
Choose Hosting → Manage → Add Website:
Then insert the domain name, set the password (it will be a password for FTP access), click Add Website:
In case you received a pop-up like this one:
Make sure to check this article.
Adding domain as an add-on is not the same as getting ownership over the domain. You need to purchase a domain separately.
If you want to use your own domain as an add-on domain, make sure it's pointing to our nameservers.
Check our article about the differences between Subdomain, Parked Domain, and Add-on Domain.