There are several ways to do this, most of them are *very* insecure. You can search Plesk forums and also this forum for those...
One way that seems safe is this:
1) create vhost.conf for the domain you need to preview. Put something like this inside:
ServerAlias sub.domain.com
2) create a DNS record under domain.com:
sub.domain.com A 1.2.3.4
3) restart Apache:
'/usr/local/psa/admin/bin/my_apci_rst'
- replace 1.2.3.4 with the IP used for the name-based hosting on your server
- replace sub.domain.com with whatever temporary subdomain you want to give to your customer
Domain.com should of course be your main domain. It really doesn't matter which one is it, as long as it's hosted on your server and its DNS resolves properly. This new subdomain should work instantly after the Apache restart.
Downsides: it isn't automated.
I don't know of any known security issues with this, but use at your own risk. Also, don't forget to remove this after the real domain has propagated - if you don't, the customer can actually steal traffic by linking his files trough your main domain
Ales