Trouble adding custom sitemap to index

I have been attempting to add a custom sitemap I am generating on the fly with PHP, to Yoast’s generated sitemap. However, without much luck so far. Found the following article but the solution doesn’t appear to be working for me:

add_filter( 'wpseo_sitemap_index', 'add_sitemap_custom_items' );
function add_sitemap_cutom_items() {
	return '<sitemap><loc>https://website.com/file.php</loc></sitemap>';
}

Any advice on what might be the matter?
Thank you.

• This topic was modified 8 years, 6 months ago by justanotherdev.