Through the Plesk control panel, you can install a Let's Encrypt SSL certificate for your domain. The certificate will automatically renew every 3 months unless it is issued as a wildcard.
Here are the steps to install a Let's Encrypt SSL certificate:
Log in to the Plesk control panel and go to the "Home" page. Click on "SSL/TLS Certificates".
Click on "Install" under the "Free SSL/TLS Certificate from Let's Encrypt" section.
Select both the domain and the www.domain checkbox and click "Get it free".
If the domain points to this server where Let's Encrypt is being installed, the installation is complete. However, if the domain points elsewhere, the installation will fail, and you need to first point the domain to this particular server using DNS management.
NOTE: In order to install the free Let's Encrypt certificate, you need to have a Webhotelli plan.