To use SSL Certificate in Plesk, please follow the below steps,
Login to Plesk Administrator using your password.
Go to Tools & Utilities.
Go to SSL Certificates in the Resources group. If you already have SSL Certificates it will be displayed there.
Click Add SSL Certificate.
In the Certificate properties, specify the details of Certificate name, encryption level, your location, organization name, host name and your email address. The details given in the certificate properties will be used to generate your private key, so make sure they are all accurate.
Click Request to complete.
If you have your SSL certificate then save it in Setting->SSL Certificates. Now choose the certificate you saved and click Secure Control Panel.