When using cPanel/WHM, you can enable the auto SSL on the cPanel account.
When this is already enabled on the cPanel account, it may happen that the SSL is not generated due to errors.
In cPanel account, go in SSL/TLS Status to see if the SSL certificate has been generated for the domain or subdomains.
In case of error, you should have detail on the reason why it was not generated.
The most frequent error is related to the .htaccess that forbid the AutoSSL to validate the domain.
It must have access to the .well-knwon directory to validate the domain and URL.
When the autoSSL is correctly installed in the cPanel then you can update your JMS slave site definition to add the https:// to your list of domains.
So if you have already define a slave site with
domain.com
www.domain.com
then add
domain.com
www.domain.com
If you want to force the redirection to the https, this is also possible when you enable the "redirect to the first one"
In this case, place the https line as the first one in the list of domains.