The on TABCOMP event triggers when you press the TAB key in an editbox and mIRC is about to perform tab completion.
Format: on <level>:TABCOMP:<*#?=!@>:<commands>
Example: on 1:TABCOMP:#mIRC:/echo event: $event target: $target line: $1-
Halting this event prevents default tab completion.
Note: This event is not triggered in an empty editbox if you have the Tab key changes editbox focus option enabled.