Is it possible to create and use Wildcard SSL certificates in Plesk?
Plesk allows to create and use certificates for Wildcard domain names (like *.example.com). Wildcard certificate can be created the same way that usual certificate is generated. The only difference is that the name with "*" sign in the left path, for example "*.example.com", will match all this domain's subdomains, like mail.example.com, ftp.example.com, etc.
Note, a certificate has to be manually installed via Plesk for all of the required subdomains.
More information about installing SSL certificates is available in Plesk Administrator Guide