ULX doesn't bring up a full steam browser, BUT, to bring up a URL, copy your server gmod/addons/ulx/data/config.txt to your gmod/data/ulx folder.
Then open it using a text editor, and change the motdfile setting from ulx_motd.txt to <the url you want to load>
Gmod's/ULX MOTD code may be broken at the moment, AND, it doesn't do well with dynamic code within the page (expandable div tags, etc)
Source ingame limitation I think, probably for security reasons.