Hyperlinking Featured Images to External Pages

I’ve seen versions of this dilemma solved here but none of the code in previous threads matches that of the theme I’m using, which is Total by WPexplorer.

My website: http://www.drunkMall.com

The homepage has infinite scroll so users can browse the contents of the entire site on one page if they choose. Currently, users who click a featured image, post title or “View Post” button are taken to an item’s full blog post, which contains an external link to the item’s purchase page.

I want for users who click a featured image on the homepage to be taken directly (via external link) to the product purchase page. I know this can be done by adding a Custom Field to the “Edit Post” screen, which I’ve already done, and altering the theme’s code with a command to add a link using the info in the Custom Field. The last part is what I need help doing.

The developer of my theme has told me which file needs to be altered. Here is the entire code in that file:

<?php
/**
 * Custom Post Type Entry Media
 *
 * @package Total WordPress theme
 * @subpackage Partials
 * @version 3.3.0
 *
 * @todo Add support for post featured video
 */

// Exit if accessed directly
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

// Thumbnail args
$args = apply_filters( 'wpex_'. get_post_type() .'_entry_thumbnail_args', array(
	'size'          => 'full',
	'alt'           => wpex_get_esc_title(),
	'schema_markup' => true
) );

// Get thumbnail
$thumbnail = wpex_get_post_thumbnail( $args );

// Display featured image
if ( $thumbnail ) : ?>

	<?php
	// Get overlay style
	$overlay = apply_filters( 'wpex_'. get_post_type() .'_entry_overlay_style', null ); ?>

	<div class="blog-entry-media entry-media clr <?php echo wpex_overlay_classes( $overlay ); ?>">
		<a href="<?php wpex_permalink(); ?>" title="<?php wpex_esc_title(); ?>" rel="bookmark" class="custom-posttype-entry-media-link <?php wpex_entry_image_animation_classes(); ?>">
			<?php echo $thumbnail; ?>
			<?php wpex_overlay( 'inside_link', $overlay ); ?>
		</a>
		<?php wpex_overlay( 'outside_link', $overlay ); ?>
	</div><!-- cpt-entry-media -->

<?php endif; ?>

The name of the Custom Field I’ve created for entering the external link is ExternalURL. How do I wrap the featured image container with a link to the webpage entered into ExternalURL?

I’m using a child theme.