This article will explain how to add additional IP addresses in Plesk. - You need to have had the IP purchased and applied to the server before this
Login to your Plesk panel.
Click on the Tools & Settings option under Server Management.
Click on the IP Addresses link under the Tools & Resources section.
Click on the Add IP Address button.
Enter the IP address and subnet mask which can be found by clicking the IP tab and looking under the IP netmasks section in your 123-reg control panel.
Choose if this is a Shared or Dedicated IP address and you can also add an SSL certificate if you have one installed.
Click on the OK button to finish added your IP address.