// Add form
function wpse261260_add_checkbox_thumbnail ($content) {
    global $post;
    $text = __( 'Link to full image', 'your-textdomain' );
    $id = 'link_to_featured_image';
    $value = esc_attr (get_post_meta ($post->ID, $id, true) );
    $label = '<label for="' . $id . '"><input name="' . $id . '" type="checkbox" id="' . $id . '" value="' . $value . ' "'. checked( $value, 1, false) .'> ' . $text .'</label>';
    return $content .= $label;
    }
add_filter ('admin_post_thumbnail_html', 'wpse261260_add_checkbox_thumbnail');

// Save form
function wpse261260_save_checkbox_thumbnail ($post_id, $post, $update) {
    $value = 0;
    if (isset ($_REQUEST['link_to_featured_image'])) {
        $value = 1;
        }
    update_post_meta ($post_id, 'link_to_featured_image', $value);
    }
add_action ('save_post', 'wpse261260_save_checkbox_thumbnail', 10, 3);