How to enable Let’s Encrypt free SSL certificate for your domain:
- Log in to Plesk.
- On the (left) sidebar, click Websites & Domains
- Click on the Let’s Encrypt symbol to pull up the Let’s Encrypt SSL Certificate page.
- Type a valid e-mail address in the box.
- Select the “Include www.(example.com)” as an alternate domain name check box. So that the SSL certificate protects your domain with and without the www prefix.