Good question. I'm surprised we don't have it mentioned in our ulx_readme.txt file.
Copy the /addons/ulx/cfg to your gmod/ root.. Answer yes to when it asks about cfg folder already existing.
Once that's done, go into gmod/cfg/server.ini - that is ULX's ini file, not to be confused with server.CFG, which is Source server settings file.
Once in server.ini, you'll see all the various things you can add/edit/set up on server start/map change.
Additional tip.. if you have special items you want set for special maps, see /gmod/cfg/maps and create a <mapname>.ini
If you only want it to advertise until next map change/server restart, see "ulx help". I think the command is "ulx addCadvert". Don't hold me to that.