Just try? That's a 30 second task.
This.
And if you can't but you want to force people using your patch to only connect to your realmlist, then you can set the C var on start up of the client via FrameXML edits.
Since C var changes take a client restart to take effect, you can get it first to see if it is correct. If it isn't, don't let them login and show a error message saying to restart the client.