You can filter the slug creation on `wp_unique_post_slug` hook.

    //Register the filter
    add_filter('wp_unique_post_slug','prefix_the_slug',10,6);

    function prefix_the_slug($slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug){
        //Get value from the custom field
        $prefix = get_post_meta($post_ID,'cw_event_start_date',true);

        //Only prefix certain post type and if prefix field existed
        if($post_type==='post' && $prefix && !empty($prefix)){

            //Prefix only if it is not already prefixed
            preg_match ('/^\d\d\d\d\d\d/', $slug, $matches, PREG_OFFSET_CAPTURE);
            if(empty($matches)){
                return $prefix.'-'.$slug;                       
            }
        }
        return $slug;
    }