If you are looking to run your very own WordPress multisite and you have chosen plesk as your server dashboard then this information is going to be especially useful for you. During the time it took to put our Network together there have been many obstacles, errors, and moments of frustration. However, the power of persistent Google usage never fails along with testing and determination.
Let's get to the juice of this article. You have a WordPress multisite setup. You have sub-sites in this multi-site setup that are using subdomains. You have a domain that you want to map to the subdomain. To do this you will need to go into the domain section of plesk and add a domain name as an alias. Depending on do you use case for the domain that you added as an alias you may or may not want to check the email option.
After you have created your domain Alias you will want to go into that domains pH P settings and ensure that it is pointed at the same root directory as your main website. Ours was in httpdocs.