Example of code to add a dynamic url for sitemaps in WP multisite / Multidomain

add-sitemaps.php

add_filter( 'robots_txt', 'thfo_add_sitemaps');

function thfo_add_sitemaps( $output ){

	$sitemap_url = 'sitemap: ' . get_bloginfo('url') . '/sitemap.xml';

	$output .= $sitemap_url;

	return $output;
}