You can install your own SSL as long as you are a dedicated server owner and have the Admin login to Plesk.
- Login to Plesk.
- From the main menu, click Domains.
- Select the domain name you want to install on.
- Under Additional Tools, click the SSL Certificates.
- Now you can request a CSR and upload your certificate attachments (provider by your SSL issuer). Alternatively, you can paste the certificate codes instead of uploading the files.