This could be useful for someone who's a theme shop on his site and would like to add a link to a demo. Instead of selecting the theme via the theme selector, you can just click the link, which is a lot easier.
Code:
<a href="{$theme_redirect_url['location']}?my_post_key={$mybb->post_code}&theme=5&{$_SERVER['QUERY_STRING']}">Test</a>
5 is the theme ID, so change it to your likings.