You can park a domain in your Cpanel control panel under the "park domain" link. Please review the cpanel manual for detailed instructions.
http://www.iccmhosting.co.uk/know/cpanel.php
Before you can create an add-on domain you must ensure the nameservers for the domain name are the ones provided for your hosting plan. You will not be able to create an addon domain if the DNS for the website you are trying to add hasn't propagated.
You can also multi-host an addon domain from your control panel. Detail instructions are provided in your control panel.
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