I'ss not clear what you are asking. I think you are trying to ask: How do you access a website before the domain registeration goes into effect? (This means that you've created a doamin in Plesk, registered the domain, created a website but the domain registeration has not propagated to the all of the root DNS servers.)
This is fairly simple, just create an entry in your hosts file (Windows XP/2000: %systemroot%/system32/drivers/etc/hosts) (Linux: /etc/hosts) for the domain with the IP address assigned to it in Pleask. E.G. 126.96.36.199 [www].[insert domain name].[com]
Just don't forget to take it back out later.
The brackets are to dodge an error in the fourm code that thinks I'm trying to link a website.