How can I return all posts that match a specific taxonomy using <?php get_search_form(); ?>

For example: if a user enters a search query that matches the name of a tag or category, search.php will return all posts with a matching tag, or that fall into a matching category. I am working with a custom post type, and as of now, get_search_form(); seems to only return matches based on content and title alone. Please Help! Thanks.