Since version 8.0.1 Plesk allows to create and use certificates for Wildcard domain names (like *.domain.tld).
Wildcard certificate can be created the same way that usual certificate is generated. The only difference that you specify the name with "*" sign in the left path, for example "*.domain.tld" domain name in the Wildcard certificate will match all this domain's subdomains, like mail.domain.tld, ftp.domain.tld, etc. Please note, that you need to install certificate manually via Plesk for all of the required subdomains.
More information about installing SSL certificates are available in our documentation