All the search boxes are "type ahead." It may take a second or two to get started, but once it gets started the search results refine as you type.
On the Overview page for most forums is a search box. You can't miss it. This searches within the specific forum you are in.
Some forums have subforums. If you are on the "parent" forum, a search at that level includes all subforums. This screenshot is from the DevGurus community.
If you want to search the entire AMD Community, go to the search field at the very top right of the window. Hey, sometimes you want to scour everything.