If you opt to purchase a 3rd party ssl you will need to instaill this yourself in plesk.
1) Step one is to raise / generate your CSR. Log into plesk and from Websites and domains select SSL/TLS Certificates
Select Add SSL/TLS certificate Give the SSL a title eg ev2017 Complete the information accurately. This will be encoded into your CSR / SSL certificate. Pay close attention to your domain name.. are you purchasing an ssl for www or non-www? Enter accordingly. Some SSLs will cover both www and non-www in which case enter non-www and the subdomain of www will be added by your provider. If you are creating an ssl for a sub-domain you will need to enter this.
2) Now you need to copy the CSR and send to your provider.
You will now be returned to the SSL screen. Click on the ssl you have just created and you will see the CSR. Copy the text and provide this to your SSL provider.
3) Enter your SSL / CA bundle
Your ssl provider will then provide you with your SSL certificate and your CA bundle in the form of a block of text similar to the CSR. Cut and paste these into the two available boxes and save,