Is it possible to create and use wildcard SSL certificates in Plesk?
How to configure Plesk to automatically assign a domain wildcard certificate for new subdomains?
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.
When a subdomain is created within the same subscription, then there is no need to manually add wildcard certificate to the subdomain. The wildcard certificate of main domain is automatically assigned to the newly created subdomain.
More information about installing SSL certificates is available in Plesk Administrator Guide