You can install free Let's Encrypt SSL with just one-click:
A. Inside your wpSTK Control Panel navigate to:
Websites & Domains > Domain.com > SSL/TLS Certificates
Click on "Get It Free" button from the Let`s Encrypt menu.
Once you have issued "Let`s Encrypt" certificate it will auto-renew before it expires.
B. In case you want to add another type of SSL certificate, navigate to:
Websites & Domains > Domain.com > SSL/TLS Certificate
Click on the "Upload" button and select your SSL .pem file.