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>