This is a quick step-by-step walkthrough for installing a Let's Encrypt SSL certificate via Plesk Onyx.
Let's Encrypt SSL certificates are free to setup and install. For more details please see letsencrypt.org.
Installing a Let's Encrypt SSL certificate in Plesk Onyx
Login to your account
Firstly you'll need to login into your Plesk 12.5 control panel, which will be at a URL similar to https://your_server_number.xenserve.com:8443/
Once you have logged in, click on the 'Subscriptions' option, in the left hand menu.
Choose Your Site
This will bring up a list of websites you have on your Reseller Hosting account.
Locate the domain name you wish to access, then simply click on it.
Choose 'Let's Encrypt'
This will bring up a an overview of your selected domain. Click on the link 'Let's Encrypt'.
Please note: If you cannot locate the Let's Encrypt icon, you may need to read Installing the Let's Encrypt SSL extension.
Install Let's Encrypt
This will bring up the Let's Encrypt page. On this page, simply click the button labelled 'Install' to install a Let's Encrypt certificate.
Installing the Let's Encrypt SSL extension in Plesk Onyx
Download the Lets Encrypt extension
If you do not have the option to install Let's Encrypt via Plesk, you will need to download the Let's Encrypt Plesk extension.
Firstly, you need to download the Let's Encrypt Plesk 12.5 extension, which you can do here.
Simply follow the above link and download the zip file to a memorable location on your computer.
In Plesk, click on the 'Extensions' option, in the left hand menu.
Choose 'My Extensions'
The click on the 'My Extensions' option, in the left hand menu.
Choose 'Add Extension'
To add the Let's Encrypt extension you previously downloaded, simple click the button labelled 'Upload Extension'.
Upload New Extension
On the upload extension form, simply click the button labelled 'Browse', and locate the Let's Encrypt extensions zip file you downloaded earlier.
After you have selected the Let's Encrypt extensions zip file, simply click on the button labelled 'OK' to install the extension.
Once the installation is complete, you will be returned to the Extensions page, where the Let's Encrypt extension will be listed. From here, please read Installing a Let's Encrypt SSL certificate to install a certificate on a specific domain.
For details on how to force traffic over HTTPS, please read our KB article titled 'How to force HTTPS using the htaccess file'.