The guidance below show how to secure Plesk interface with a free Let's Encrypt certificate:
Log in to Plesk.
Go to Tools & Settings > SSL/TLS Certificates (under Security).
Click Let's Encrypt.
Fill in the Domain name and Email address fields and make sure contain valid information:
* The hostname/domain name must be resolved to a public IP address of the Plesk server from the Internet.
Click Install. An SSL certificate from Let’s Encrypt is generated for the hostname and set to secure Plesk on port 8443. This certificate will be auto-renewed every 90 days. Here is the final look:
Now, can access Plesk over https://server.example.com:8443.