If your Windows hosting package includes more than one domain name (website), then you can easily add domains to the server. Before you start adding a new domain that will use a second-level domain name, like example.com, be sure to register this domain name. You can register the new domain.

To host a new website with an addon domain name:

1. Login to Plesk Control Panel.
2. Go to Websites & Domains.
   
3. click Add Domain.
   

   Note: If you select the recommended option, you should connect the DNS settings related and your domain name with the domain name at the registrar's side. To achieve this, enter the Plesk name server IP address into the respective NS record of the registrar. The instruction on how to obtain the IP is as follows: go to Websites & Domains > domain name > DNS Settings, find the NS record, and then find the A record corresponding to the NS record value.
   For example, if your NS record isexample.com. NS ns.example.com,
   find the A record with ns.example.com, for example, 
   ns.example.com. A 192.0.2.12
   The resulting value, 192.0.2.12, is the Plesk name server's IP that you need.

4. Specify the name of the Addon domain or the new domain that you want to add in your web hosting account on this page.
   
5. Select the type of hosting you want to create. If you want to host a website for this domain, select Website hosting.
6. Just below that, you can see DNS settings. There you can select from the options whether you want to use local DNS server or external DNS server for the domain you are creating.
7. Then you can see Mail Settings. If you want to use email service for this new addon domain please select this option.
8. Then below, under the title Hosting Settings, Document root enter the Path for the website home directory.
9. Click Ok.

The new addon domain name is created and now shown in the list at the bottom of the screen. You can now start to upload your web content to the web space of the new addon domain website.

As described in the article, If you want to delete or remove addon domain from your hosting plan in Plesk.