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). A wildcard certificate can be created the same way as usual certificate is generated. The only difference is that the name with the "*" sign is specified in the left path. For example, "*.example.com", will match all 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 a wildcard certificate to the subdomain. However, the wildcard certificate of the main domain still needs to be manually assigned to the newly created subdomain at Plesk > Domains > one.example.com > Hosting Settings > under Security, select the "Let's Encrypt example.com (one.example.com)" certificate > click OK.
Instructions to get a free wildcard certificate are available here.
To create Let's encrypt wildcard certificates, see instructions here.