Why is my host-mapped Zendesk subdomain not appearing in search results? How long does it take for Google to reindex my new host-mapped subdomain?
If you have just set up the host-mapped feature, it might take up to a few weeks for your new URL to be reindexed by Google and for the new URLs to be displayed in Google's SERPs. You can read more about Google's Indexing process in their documentation here.
If you have submitted the new address of your host-mapped Help Center via the Google Webmasters Tool, then you can use the Fetch as Google feature which allows you to to test how Google crawls or renders a URL on your site.
If you want to test if your new host-mapped subdomain has been reindexed by Google and how many pages have been indexed at any given point, you can use the following Search operator in Google: site:yournewsubdomain.yourdomain.extension.