But how would I send messages to it?
Exactly the same as sending messages to any other IP -- the difference is that any message send to the broadcast IP will be received by *all* computers on the network, including your server.
you'd need to come up with some kind of 'handshake', where if the server receives the broadcast message it will open a connection to a specific port on the sending PC where it can communicate some of the settings such as the server's actual IP address.
Keep in mind that any info sent to the broadcast IP is by design visible to any computer on the network, so don't send any confidential info such as username/password stuff there. Just use it for a handshake so the server and client can then make a direct connection, and send login info over *that* connection instead.
By the way: thanks for the link to that code archives bit to get the broadcast IP. Too bad it requires an external DLL. :-?