Thank you for the update! We will consider this functionality in upcoming releases if it will be popular.
Everyone, please continue voting for this feature if you consider it important.
There are over 10 pages of people asking for this on this site according to Google:Anonymous commented
Why should the sub-domain be forced to webmail?
The purpose of Plesk to t allow the administrator to easily manage their server. These sorts of choices should not be hard-coded.