Try this link
http://www.webbmotell.nu/PLESK/manual/ch19.html#N860d
Notes on Certificates:
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In order to use SSL certificates for a given domain, the domain MUST be set up for IP-Based hosting.
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When an IP-based hosting account is created with SSL support, a default SSL certificate is uploaded automatically. However, this certificate will not be recognized by a browser as one that is signed by a certificate signing authority.
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The default SSL certificate can be replaced by either a self-signed certificate or one signed by a recognized certificate-signing authority. The self-signed certificate is valid and secure, but many clients prefer to have a certificate signed by a known Certificate Signing Authority.
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If using a SSL certificate issued by a certificate authority other than Thawte or Verisign, a rootchain certificate is required to appropriately identify and authenticate the certificate authority that has issued your SSL certificate.
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If the given domain has the www prefix enabled, you must set up your CSR or self-signed certificate with the www prefix included. If you do not, you will receive a warning message when trying to access the domain with the www prefix.