How to link the WordPress title & thumbnail to an external URL.

content-archive.php

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

	<header class="entry-header">
		<?php
		if ( has_post_thumbnail() ) :
			the_post_thumbnail();
		else :
			print_post_no_image_thumbnail();
		endif;
		?>

		<?php
		if ( is_singular() ) :
			the_title( '<h1 class="entry-title">', '</h1>' );
		else :
			print_post_title();
		endif;
		?>
	</header><!-- .entry-header -->

</article>

functions.php

/**
 * Add Dashicons
 */
function load_dashicons_front_end() {
	wp_enqueue_style( 'dashicons' );
}
add_action( 'wp_enqueue_scripts', 'load_dashicons_front_end' );

/**
 * Generate the post title link tag to external url with icon.
 *
 * @return void
 */
function print_post_title() {
	$id        = get_the_ID();
	$title     = get_the_title( $id );
	$key_value = get_post_meta( $id, 'external_url', true );
	$icon      = '';

	// Check the custom field value.
	if ( ! empty( $key_value ) ) {
		$path      = $key_value;
		$extension = pathinfo( $path, PATHINFO_EXTENSION );

		if ( 'pdf' === $extension ) {
			$icon = '<span class="entry-icn dashicons dashicons-media-default"></span>';
		} else {
			$icon = '<span class="entry-icn dashicons dashicons-external"></span>';
		}
	} else {
		$path = get_permalink( $id );
	}

	// @link https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/wiki/Whitelisting-code-which-flags-errors
	echo '<p class="entry-title"><a href="' . esc_url( $path ) . '" rel="bookmark" title="' . esc_html( $title ) . '">' . esc_html( $title ) . '</a>' . $icon . '</p>'; // WPCS: XSS ok.
}

/**
 * Generate the post no image thumbnail tag to external url.
 *
 * @return void
 */
function print_post_no_image_thumbnail() {
	$id        = get_the_ID();
	$key_value = get_post_meta( $id, 'external_url', true );

	// Check the custom field value.
	if ( ! empty( $key_value ) ) {
		$path = $key_value;
	} else {
		$path = get_permalink( $id );
	}

	echo '<a class="post-thumbnail" href="' . esc_url( $path ) . '" aria-hidden="true" tabindex="-1">';
	echo '<img class="attachment-post-thumbnail" src="' . esc_url( get_template_directory_uri() ) . '/img/news-no-image.png">';
	echo '</a>';
}