I want to add a disclaimer pop-up dialog box that all users must acknowledge before being able to navigate the rest of the portal.
How might this be possible in the most simple way? Can it be done utilising the out-of-the-box Portal for ArcGIS install? Is there a tutorial somewhere?
This is possible now in ArcGIS Enterprise 10.8 by using the access notice. You can choose the text that is displayed and users will need to accept the notice before being granted access to your ArcGIS Enterprise portal.
More on the access notice is here in the What's New in ArcGIS Enterprise 10.8: Portal Administration blog.