Here's how:
net_start
sv_lan 0
deathmatch 1
maxplayers (whatever you want)
map (mapname)
restart

it will add you to the master server and it works. I know, it looks like the leak, but I assure you this is the retail HL2.

We're running a bit of an experiment. We're keeping track of the accounts that do this and will be shutting them off.