clientcmdMessageBoxOKCancel does not exist. The only one is clientcmdMessageBoxOK.
The generic clientside functions messageBoxYesNo(); and messageBoxOKCancel(); exist, but they cannot be called without installing an Add-On, which the topic starter does not want. They weren't included because the general function allows any piece of code to be executed - if made available to servers people could make unavoidable (generally) "Message Box OK: Crash / Delete All Files?" messages to destroy peoples' games.