This article applies to 123-reg servers running Plesk.
This article will explain how you can map an additional domain to a hosting account on Plesk.
Login to your Parallels Plesk Panel.
Click on Subscriptions link on the left hand panel.
Click on the Open in Control Panel link for the domain you wish to access.
Click on the Website & Domains tab.
Click on the Add New Domain Alias link.
Enter the domain alias you wish to add and select the domain it will be mapped to from the drop box, choose what settings you wish to add the new alias to. Press the OK button to complete the process.
Please note: You will need to point the mapped domain to your VPS using DNS.