Authentication window does not work in a world of "Let's Encrypt"

This is on an SSG5:

For years, I have been limiting Internet access from a couple subnets via the authentication option.  The users would have to enter a valid local userid & password to get out.

But now in the world of "Let's Encrypt", all the sites my users go to are https which does not trigger the authentication.  And the nice browsers substitute just the url (if entered) with https://url.

I just upgraded to the latest firmware:  6.3.0r26 and still no https support for authentication.

I followed

https://forums.juniper.net/t5/ScreenOS-Firewalls-NOT-SRX/AUthentication-Window-does-not-pop-up/m-p/4335#M1797

And made a service group including https and http (and ftp and telnet and...) and still no authentication window.

So what can I do other than to find a new site that has not switched to https (yet) and have the users go there first?