Hey Eduardo,
As far as I can see in this particular case the browser is calling an actual system dialog with secure entry fields, so we've stepped out of the browser and into the system UI.
Blame Apple and complain to them (file feedback).
I don't use JXA much for that reason, although I've been wanting to get more into it due to the rich feature-set of JavaScript.
See this thread for some hints: Learning & Using AppleScript & JavaScript for Automation (JXA)
-Chris