Shortcode displays above the post content

Hello everyone, I am trying to add a shortcode (I’m doing this for the first time in my life). The content of the shortcode shows up above the post content, and I want it to be at another place (below the post content).

I already read this and this topics, and so far I have the following code, which hopefully doesn’t crash my site, but still shows up above the content (in the wrong place):

function simplybook_script () {?>
<script type="text/javascript" src="http://wanadiis.simplybook.me/iframe/pm_loader.php?width=700&url=http://wanadiis.simplybook.me&theme=&layout=widget&timeline=modern"></script>
<?php
return;
ob_start();
add_simplybook_widget ();
$output_string=ob_get_contents();;
ob_end_clean();
return $output_string;
}
add_shortcode ('simplybook_widget', 'simplybook_script');

Please help what’s wrong with the code? Thank you in advance!

[bump moderated]