Secure Webmail is enabled at Plesk > Domains > example.com > SSL/TLS Certificates
Why there is no redirection from http to https if enter webmail.example.com into a browser ?
After Secure Webmail is enabled the HTTPS connection will be used only if to go to Mail tab and click the webmail icon for an email address. webmail.example.com will be opened in new window via https. There is no actual redirection from http to https for webmail.
If it's required to force webmail to use https article How to redirect webmail HTTP to HTTPS can be applied.