SMTPS and Submission are email protocols for encrypted authentication for outgoing email. By default, they use ports 465 and 587.
To set them up, you'd first need to add an SSL certificate for one of your Virtual Servers. Virtualmin can then copy that SSL certificate into Postfix for SMTPS and Submission.
To generate a self-signed SSL certificate, you can simply go into Edit Virtual Server for the domain in question, and set SSL Website Enabled.
Then, click Server Configuration -> Manage SSL Certificate. From there, you can click Copy to Postfix in order to enable SMTPS and Submission with your newly generated self-signed SSL certificate.
This will of course work just fine with a commercial SSL certificate. For instructions on setting up a commercial SSL certificate, see these instructions: