I've added that domain to the account for you. Please note that it may take up to 2 hours for the domain change to take effect, and it will not work until you set up the DNS with the domain registrar.
To configure the DNS, please see the steps provided on our Wiki to either set NS records pointed at the HelioHost nameservers, or create A/AAAA records and point them to the server's IPv4/IPv6 address: https://wiki.helionet.org/Addon_Domains#Custom_Addon_Domains
If after a full 2 hours it doesn't work on your side, please make sure you clear your web browser cache: https://wiki.helionet.org/Clear_Your_Cache