Click their profile icon, view activity, it will take you to where it says their profile is hidden.
Copy the user battletag from the URL.
Heres mine as an example:
https://us.forums.blizzard.com/en/d3/u/TheDarkJedi-1970/activity
Click your user profile icon (upper right), click the bell → Preferences → Users (left) → Add… Enter the battletag.