By default, CommandBox will open your browser with the host and port of the server. You can customize the exact URL that opens. This setting will be appended to the current host and port.
Or you can completely override the URL if your setting starts with http://.
For Multi-Site, the openBrowserURL can be configured on a per-site basis in the sites object of the server.json or in a .site.json file. The browser will not open automatically, but this setting will control the options in the tray menu and the server open command.