It's off by default.
From the help display (". #help"):
dotcom by Natasha
. <channel> #on Turns the given channel on.
. <channel> #off Turns the given channel off.
. <channel> <say> Says the given message.
. <channel> :<pose> Poses the given message.
. <channel> #who Lists people awake on the given channel.
. <channel> #all Lists everyone on the given channel.
. #list Lists the existing channels.
. #mine Lists the channels you're listening to.
. #help2 More dotcom options.
(". #help2")
. #ignore <name> Stop receiving messages sent by <name>.
. #!ignore <name> Undo '#ignore <name>'.
. <channel> #lock [<lock>] Locks the given channel, or displays the
current lock if one given. See 'help @lock'
for <lock>'s syntax.
. <channel> #unlock Unlocks the given channel.
. <channel> #prepend <string> Sets your prepend for the given channel.
. #prepend <string> Sets your default prepend.
Put timestamps (see 'man timefmt'), ANSI color, and '%c' for the channel name
in your #prepend.