Select the Websites menu option from the Dashboard sidebar. 

Click on your website's name to go to the website overview.

Click the Domains tab to view all mapped domains for your website.

Locate the domain you want to issue a certificate for and click Configure → Change Behavior.

Select Yes for Enable HTTPS.

Select Free by Let’s Encrypt for Select type of SSL Certificate.

Click Confirm to save the changes. A background job will be initiated to generate and install the certificate on the website.