Can I edit the robots.txt file that my Zendesk subdomain is using?
The robots.txt file is a shared file used by all Zendesk accounts. It is not account-specific and cannot be edited.
Currently, the only option to block a page from being indexed is to add a meta tag to your Help Center.
To temporarily exclude a specific page from being indexed by Google, see How do I exclude a Help Center page from being indexed by Google?.