You can use the TAB key in the Editbox (and in various popup dialogs like $? $input to complete various words)

TAB - Completes and cycles all channel nicks, if current window is a query window, the nick of the query is used.
nick TAB - Completes channel nicks, query nick or your own nick.
$ TAB - Evaluates a identifier, eg. $me TAB $chan TAB.
% TAB - Evaluates a global variable.
# TAB - Completes and cycles all joined channel names (and any channels in your Serverlist for the current server), if current window is a channel, the channel name is completed first.
#chan TAB - Completes a joined channel name.
<@|%|+>nick TAB - Completes channel nicks matching the channel prefix and/or nick. (Based on channel prefix received by the server, e.g @+)

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