The search bar at the top right is what you should use.
For example, I am in the Desktop Video forum board, if I search, "XPS 8500, video card upgrades" (without the quotes), a drop down appears with all threads using that search string. I then click "Show All.." and it shows 88 threads.
In the upper right-hand corner of the page is a search box, ... but I don't use that to search the forum.
I use Google to search the forum, and use the "site" prefix to limit the search.
If you want to try it, copy paste the following line into the google search box, and you'll see an example. copy it exactly including the double quotes:
site:en.community.dell.com "search a forum"
You'll probably see that this post is at the top of the search results.
The other benefits of using this are there are a bunch of other advanced search options, and they are really well documented across the internet. for example: www.lifewire.com/advanced-google-search-3482174