How to add post featured image url to inline background url()?

You just need to retrieve the src attribute of the thumbnail:

$thumb_src = wp_get_attachment_image_src ( get_post_thumbnail_id('thumbnail'));

$thumb_src is then an array containg the url, width and height of the thumbnail image. So something like…

<div style="background: url(<?php echo $thumb_src[0];?>
) !important;">adasdasdasd </div>

should work.