How can my end be automatically signed into my Help Center automatically when they visit it after signing into my external site?
It is possible to set up your Help Center to use the same authentication method as your external site by setting up SSO. However once the end-user visits your Help Center, in the same session as they have signed into your external site, they will still need to use the sign-in option on the Help Center to be signed in there too.
In order to remove this extra step, if you configure your Help Center so that it requires sign-in to access, when visitors are signed into your external site in the same browser session, the Help Center will carry across that authentication automatically when they visit and they should be signed in without any effort on their part.
If you don't want to require sign-in to access the entire Help Center, then, when you link to your Help Center from your external site, link to a section that does require sign in, e.g. an article that is restricted to signed-in users only. When the user is directed to that page, the authentication should kick in, signing them in automatically, and they should just be shown the article without having to sign in manually. This option will not work in an open Help Center if the user just navigates to your Help Center's home page directly i.e. subdomain.zendesk.com/hc.