Documentation

this hook allows the registration of Custom Post Types in Subscribe2 so that notifications can be sent for new posts created in such a custom post type. This hook may need to be used with the ‘s2_taxonomies’ hook details next.

function my_post_types($types) {
    $types[] = 'my_post_type';
    return $types;
}
add_filter('s2_post_types', 'my_post_types');