So you’re trying to setup a multisite WordPress blog network and have trouble setting up a domain directly to the child blog AND you’re using PLESK?
I had the same problem and found an easy solution myself.
It seems like there is a lot of stuff online about this topic but mostly outdated or not directly for PLESK so I’ve decided to show you here how you can make the multisite work with one domain for every sub blog with the help of PLESK.
I am using here the plugin WordPress MU Domain Mapping.
Set it exactly up as the author says in the install notes on WP Plugin website.
Care that you activate that plugin in the MAIN DASHBOARD of MultiSite WordPress.
This is crucial because the whole thing won’t work, if you accidently don’t activate that plugin in the main dashboard (herefor just click on “My Websites” in the backend toolbar at the top. You’re now in the main dashboard and can activate the WP MU Domain Mapping Plugin).
Set DNS of the domain to your webserver, as usually
First just setup your domain to your regular webserver as you usually would do, probably with CNAME or the IP of your webserver.
Nothing special here.
You should now be in the “add domain” form.
Fill this form like I did in the following screenshot, but with your data of course.
Be aware that the domain is led to the base of your WordPress multisite directory.
Now you just switch to the backend Dashboard of the sub blog and go to Tools -> Domain Mapping.
Here you enter the just set up domainname and activate it as primary.
Now the new domain should work perfectly just as without WordPress multisite.
If not clear your browser normal cache and DNS cache.