1. Go to cPanel
2. Select Let's Encrypt SSL (as shown in picture below)
   
3. Pick the domain that you want to add here.
   Please note that if you are using addon or subdomain, kindly add that domain first in cPanel > Domains.
   If you cannot find the domain listed in this page, kindly add it first to the cPanel.
   
4. Tick any option that you think is suitable for you and press "Issue"
   
5. You're done. If you still need any help, kindly contact our live chat support.