Domain Masking

Domain Masking (a.k.a. Blind Forwarding) is using a domain name to display a different domain, but still shows the original domain’s name in the address bar.

Generally, this is considered a bad idea and not good for SEO, but it is possible.
Plesk
(Applies to 10 – 11.0)

In order to forward a domain blindly, you will create it in Plesk as you would any other domain, only select the Hosting Type as Forwarding, then select “Frame Forwarding” to spoof the domain name to your your domain name.

Domains > domainname.com > Websites & Domains
Scroll to the bottom of the page and click the domain name
Locate the Hosting type and click ‘Change’
Now select Forwarding and Frame Forwarding.