I don’t remember the details, but based on my skill level I know it wasn’t anything novel. At the time I was learning my first programming language, Perl. IIRC I had a Perl daemon running on my computer that accepted an http request, did a reverse dns on the origin and sent the hostname in a net send message. Some of my coworkers used Sun workstations. I could get notifications from them but obviously couldn’t send them a net send message in response.

The js probably pinged their own server with then did the 'net send'

IE supported vbscript, though I don't know how far back that goes. You can certainly run arbitrary commands from jscript or vbscript using an hta app (or wscript)