We have a requirement to provide instructions to users of our ArcGIS Enterprise Portal prior to submitting the login form (see attached login.png). This would appear as static text, or a link to a html or pdf document.
I've found a few fields under Organization / Settings, but they all appear after the user has authenticated. Is there an option to customize the content on the login page its self?
I'm not quite sure if this can be done exactly the way you'd like, but this article is a good jumping-off point for customizing the Portal for ArcGIS website with your own custom code. I'm not certain whether the login dialog can be customized in this manner, but you might be able to customize the Portal Home screen (seen before the user logs in, unless you're using IWA or IdP-initiated sign-on) to include such a warning.