External link on featured thumbnail through custom field

I am trying to add an option to put an external link on any give featured thumbnail though the custom fields. having a little bit of trouble. here is my loop below (calls categories) THANK you in advance

<div id="app">
  <div class="title-steal"><h1>Applications</h1></div>
  <div class="title-steal"><h2>Click thumbnail to download mobile application</h2></div>
<div class="steal-loop">
	<?php $recent = new WP_Query("cat=23&showposts=10"); while($recent->have_posts()) : $recent->the_post();?>
		<a href="<?php the_permalink() ?>" >
<h2 class="feat-post-top-title-steal"><?php the_title(); ?></h2></a>
	<div class="feat-thumb">

    <a href="<?php the_permalink() ?>"><?php if ( has_post_thumbnail() ) the_post_thumbnail('steal-loop'); ?></a>

    </div>

	<?php endwhile; ?>
</div>
</div>