How to add write/modify permissions for IIS application pool user to folders and files in httpdocs folder?
To add permissions for application pool user, you should go to Plesk Control panel->subscriptions->domain.tld->hosting settings>additional write and modify permissions.
Unfortunately there is no ability to manage permissions for IIS application pool user with hosting security utility (%plesk_bin%\HostingSecurity.exe). List of available users you can find here:
Feature request for ability to manage permissions for IIS application pool user with HostingSecurity.exe is created as well. Request id is #1700620