Add one line to the special configuration file for the subdomain.
Normally this file is located in /srv/vhosts/domain/subdomains/subdomain/vhost.conf .
Create the file, if there is no file yet(vhost.conf).
Add the following line, save it and exit the editor:
" ServerAlias www.subdomain.domain.com " - without the quotes
This is a Apache configuation that says that Apache will also serve the subdomain if you access it with www. This is normally used for the domain name, but also working for sub-domains.
Recreate the Plesk / Apache configuration for the whole domain:
/usr/local/psa/admin/sbin/websrvmng -u --vhost-name=domain.com