Did you try the code exactly as Jonathan posted it and only changing the field name?
<?php
function acf_set_featured_image( $value, $post_id, $field ){
if($value != ''){
//Add the value which is the image ID to the _thumbnail_id meta data for the current post
add_post_meta($post_id, '_thumbnail_id', $value);
}
return $value;
}
// acf/update_value/name={$field_name} - filter for a specific field based on it's name
add_filter('acf/update_value/name=cursusfoto', 'acf_set_featured_image', 10, 3);
?>
