For the purpose of demonstrating this tutorial I will be using Ubuntu 18.04 LTS.
Step1. Check Webmin version
Webmin can now request SSL certificates from Let’s Encrypt from within the UI. However to use this you will have to be at least on v 1.78
Step 2. Install Let’s Encrypt Client
sudo apt-get install letsencrypt
Step3 – Configure Webmin
Goto Webmin –> Webmin Configuration
Click on the gear icon on the top left corner called the “Module Config”
Fill in the Full path to Let’s Encrypt client command field with the absolute path of letsencrypt
Goto Webmin –> Webmin Configuration and client on the box “SSL Encryption”
Click on the “Let’s Encrypt” Tab.
Click on the “Request Certificate” button
Step 4 – Completed
Refresh the Webmin interface and the site would now appear properly secured via HTTPS.