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.
If subdomains are created within the same subscriptions, then there is no need to add wildcard certificate for each subdomain. When the certificate is created, it is available for each domain and subdomain in subscription.
Note, a certificate has to be manually set via Plesk for all of the required subdomains.
More information about installing SSL certificates is available in Plesk Administrator Guide