Forum search box
The Forum search box web part displays a simple search box that allows users to look for a specified expressions in the content of forums. When a search request is submitted, the web part redirects to a page where the results are displayed by the Forum search results web part. This web part uses the SQL search engine.
The web part belongs to the Forums module. Full documentation of the module can be found in Kentico CMS Developer's Guide -> Modules -> Forums.