The situations where you'd need to insert a question mark into the CLI are pretty rare, unless working with voice functionality as in regular expressions it means zero or one instance of preceding pattern (i.e. optional). For example when trying to make a dial-peer that matches 10 or 11 digits:
dial-peer voice 16 pots
description Freephone 10/11 digits
To insert a question mark, simply press Ctrl + V prior to inputting the ?. This acts as an escape sequence & prevents the context sensitive command line help from appearing instead.