Question

When customers are adding a new domain, they have to choose existing domain in "Location of the website's files". How to create new DocumentRoot location for the new domain name?

Answer

According to Plesk business logic, all additional domains are created in the location of the main domain of subscription. It is caused by the fact that these domains have one common system user (which is defined in /etc/passwd and in 'psa.sys_users' table).
Therefore, it is not possible to set location for the additional domain out of the main domain's location. Separate DocumentRoot locations are only created for new subscriptions.
The DocumentRoot directory location for a new domain can be customized, but it will stay within the main domain location.