Featured image as a full size background in div element

In the page.php I’m trying to add a div after the header. Inside this div I want the post’s featured image as a full size background-image.

I’ve been looking at various threads, but while some were resolved, I couldn’t make it work.

This and this.

My webpage: https://rubinreklame.dk/design-til-folket/

Below is some code, where I at least get something online (the alt-text).

<div id="featuredimage">
<?php
$url = wp_get_attachment_url( get_post_thumbnail_id($post->ID) ); ?>
<img src="<?php echo $url; ?>" longdesc="URL_2" alt="Text_2" />
<?php ?>
</div>

#featuredimage{
width:100%;
height:600px;
overflow:hidden;}