To secure a web site behind the reverse proxy, configure the proxy to terminate HTTPS and handle all SSL/TLS traffic for that site. A quick, cost-free way to obtain the required certificate is to use Let’s Encrypt.
On the left menu, go to Your System > Certificates Center
Click on the button “New Certificate”
Fill the form properly and pay attention of the Common Name that represents the hostname of your Web server.
Click on “Generate Certificate” button
Your certificate settings is now saved in the list, click on the new generated certificate configuration.
Click on “Let's Encrypt Certificate” button
If you want the certificate handle other hostnames, set the list in “All Others Domains” field.
Click on the Create Certificate button
Your certificate is now stamped to “Let's Encrypt” certificate in the list, you can now affect the certificate to your reverse-proxy website.