Querying page tags

Hi!
I just used the following code to add tags to my pages:

function add_tags_to_pages() {
    register_taxonomy_for_object_type('post_tag', 'page');
}
add_action('init', 'add_tags_to_pages');

I’d like to now be able to set up a query using these tags, but I don’t see that as an option under the post query parameters. Is there a way to add this?

Thanks!
Elisabeth