If you already have a valid HTTPS certificate you can add it to your web server.
Open TSPlus AdminTool, click on the "Web" tab, go to the "HTTPS" menu and then click on "Add my HTTPS certificate" as shown in the screenshot below:
A window will open where you can select your certificate:
Your certificate file must use the ".pfx" format.
If your certificate is protected by a password, you can enter it in the corresponding field.
Click on the "Add my certificate" button to add your certificate.
Please note that you will need to renew your certificate manually.
You can convert your .pem in pfx format with Tools or online sites. For example, on this site: https://www.sslshopper.com/ssl-converter.html
You can use this SSL Server Test tool in order to validate the good quality of a web portal in HTTPS.