I don't know what method you use to trap "/" commands, but you normally use ChatWndSendMessage(Message).
Then the variable Message, is the whole message that was sent, including to bit after the parameter, so you can get all the data you want from there.
e.g.
code:
function OnEvent_ChatWndSendMessage(ChatWnd, Message) {
var checkExpCommand = /^\/command/i
if ( checkExpCommand.test(Message) ) {
Param = Message.replace(checkExpCommand, "");
}
}