saving meta/custom field to tag

you can use wp_insert_term() to create the tag and then wp_set_object_terms() to set it as the custom post tag:

//get the tag
$tag = $_POST['ad_Tags'];
//create the tag
$term_id= wp_insert_term(
    $tag, // the term 
    'post_tag', // the taxonomy
    array('description'=> 'term description','slug' => 'term-slug')
);
//set the tag
wp_set_object_terms(  $post_id, $term_id, 'post_tag' );