Snippet: Enable Post Series For Custom Post Type

/**
 * Register the post series taxonomy for other post types.
 *
 * This function register the taxonomy so you can now start using it, however it won't
 * display the actual post series on the front-end, for that I would recommend creating
 * a dynamic template for your portfolio posts and then inserting the Post Series element or
 * adding the Post Cards element with the Query Type set to "Related by Taxonomy".
 */
add_action( 'init', function() {
        // Add post series support to the portfolio
	register_taxonomy_for_object_type( 'post_series', 'portfolio' );
} );