How do I query to retrieve custom post title by tag name in wordpress

I want to retrieve custom post title by tag name.I have tag name and i want to retrieve title.How can i do this.

 <?php
        $tagname=get_post_meta(get_post()->ID, 'campaign_tag', true);
        echo $tagname;
        $args = array(
            'post_type' => 'campaign',
            'posts_per_page' => 20,
            'tag' => $tagname,
        );

        $query = new WP_Query($args);
        while($query -> have_posts()) : $query -> the_post();
    ?>
    <h5><?php the_title(); ?></h5>
    <?php endwhile; wp_reset_query(); 
    ?>