I want to make sure all my agents connect to my Deskpro On-Premise installation securely via HTTPS, but I don't see any settings about SSL in admin.
All the set up and configuration of SSL for Deskpro On-Premise is done at the webserver level. Consult your webserver documentation for details of how to set up SSL. Bear in mind you will also need to obtain and install an SSL certificate.
There are no settings you need to change in Deskpro to enable SSL, except to make sure that the PHP OpenSSL extension is installed (you can check this from Admin > Server > Server Requirements).
You can also follow our guide on how to use your own certificate which also guides you through the settings of changing from port 80 to 443 and this way enabling SSL. You can find it in Using your own SSL Certificate.