How to display the featured image of a parent page in all child pages?

Hello,

can you tell me how I would do this in GeneratePress? I tried placing this code in functions.php (in a child theme) without any luck.

function get_parent_post_thumb($post){
    if ( $post->post_parent ) {
        $parentId = end($post->ancestors);
        the_post_thumbnail($parentId);
    } else {
        the_post_thumbnail($post->ID);
    }
}

Code source: https://www.isitwp.com/get-parent-featured-image-in-children-subpage/

Thanks