Add here with masking option

Unfortunately, to be able to use your custom domain with our hosting you’ll need to change the nameservers first, as forwarding with masking is most likely blocked by this system:

The process is documented in the article linked above by @solace-ken:

After changing your nameservers to ours, you can add your domain to your hosting account, either as a Parked Domain or an Addon Domain depending on what you wish to do (check out this article for more information):

Now, you may change the nameservers back to your registrar’s default ones. You’ll need to add DNS records there to make your domain point to this hosting; the most easy way to achieve this is copying the “Website IP” value from the account section of the Client Area’s and adding it as an A record on your domain):