BBX2 has an SSI tag generator.

Just log into your admin

click < Update Pheeds > for the category you want to use

Scroll to "Generate Special SSI Tag" and fill in the basic info.

You'll get an SSI tag that looks something like this:
<!--#include virtual="/cgi-bin/BBX2/feedsearch_ssi.cgi?cat=5&keywords=kittens&mode=all&results=15" -->

This will work on all pages in your root/main folder on your site.

If your pages are in a subfolder, such as:
www.yourdomain.com/SubFolder/page.html

Then you need to tell the tag to "move up" a folder and add a couple of dots:
<!--#include virtual="../cgi-bin/BBX2/feedsearch_ssi.cgi?cat=5&keywords=kittens&mode=all&results=15" -->

Sometimes you need to add two more dots and another slash, if you're in a sub/sub folder:
<!--#include virtual="../../cgi-bin/BBX2/feedsearch_ssi.cgi?cat=5&keywords=kittens&mode=all&results=15" -->