Sorry, its not obvious how to do anything more than a simple search.
We're running Vanilla Forum. Unfortunately, the basic search seems to be it. The advanced search is only available on their hosted solutions as per https://open.vanillaforums.com/discussion/37796/advanced-search-in-this-forum
A better way I found to search in this forum is by using Google restricted to this forum.
In the Google search form, entrer : site:collectiveaccess.org/support followed by your searched words