I wanted to post this here in case others have this question.
In the new version of Plesk (Windows) I cannot find a way to give a subdomain it’s own Application Pool under a parent domain. It looks like you are forced to run the the subdomains in the same app pool as the main site if you manage them through the interface. I know how to set this up manually but would like to manage it through the interface if possible.
Plesk allow us 3 type of Application Pool Global Settings in IIS. If you select “Always assign application pool for each domain” you will be able to have one application for each domain or subdomain you add.
However, if you are trying to move subdomain in a separate pool, plesk has no option. You need to do it manually through IIS :
Run > inetmgr > You will get IIS > go to Application Pool > Find the website > Go to properties of that domain > change application pool in Home directory.