Plesk provides native support for Server Side Include scripting for SHTML, HTML, and HTM files.
If SSI scripting still doesn't work, you might need to add the following handler to the he system.webServer section of your Web.config file:
<handlers><add name="SSINC-html" path="*.html" verb="*" modules="ServerSideIncludeModule" resourceType="File" requireAccess="Script" /></handlers>