MAYBE, it will be a good idea put a Sticky on top with regular questions like this, a "FAQ" topic.
For this, you will need have your main domain working on your server.
1. Login into your server as root, then:
QUOTE
pico /home/httpd/vhosts/.skel/conf/vhost.conf
2. copy and paste this line and change "yourpreviewdomain.com" for your main domain (dont change @domain_name@.!!!!):
QUOTE
ServerAlias @domain_name@.yourpreviewdomain.com
3. press [Ctrl] + [X] then [Y] and finally [enter]
4. login in plesk as Admin, go to yourpreviewdomain.com and then to DNS and create an A record like this (change the xxx.xxx.xxx.xxx for your main ip):
QUOTE
*.yourpreviewdomain.com. A xxx.xxx.xxx.xxx
5. Done!, now all your NEW domains can ve pre-viewed as
http://domain.com.previewdomain.com/
example:
http://titeresburbujas.com.ar.elnameserver.com.ar/
The originial HOW-TO can be found here
http://forum.plesk.com/showthread.php?s=&threadid=7346