k - Keyed
Sets the channel so only users who specify the room's key (password) as the second parameter of the join command can enter the room. If the key to the room #CoolChat were bagel then to join one would use the command /JOIN #CoolChat bagel. Unlike passwords, keys are not case sensitive.
Privilege: Channel Operator
See also: CMODE j
Examples:
/mode #webchat +k mykey123