Today our crawler only crawls 1 main domain. It will ignore/exclude all subdomain links found in the crawl and not render them on the sitemap. So if crawling a subdomain is required, it will need to be a separate unique crawl.
However, you can manually add a subdomain placeholder into your sitemap via the *Editing feature. *available only on Freelancer plans and above :
1 - Create a new directory folder.
2 - Enter the subdomain URL
3. optionally ( create a tag called 'subdomain' ) to help see it on the map with an accent color.