That would be a mIRC feature.
/help Tab
Tab
If an editbox is empty and you press the Tab key, mIRC inserts a "/msg nickname" into the editbox of the window you are currently on, where nickname is the last person that sent you a message.
You can use Control+D to remove a nickname from the Tab Key nickname list of users who have recently messaged you.
Note: The editbox needs to be empty in order for the Tab key to work as described above since the Tab key also performs the functions below...
If you are in a channel window editbox and it contains some text, the Tab key performs nickname completion on the word the cursor is on, this allows you type in only the first character or two of a nickname on that channel and then press Tab and mIRC will expand it to the full nickname.
If you press the Tab key while the cursor is positioned over a %variable or $identifier, it is evaluated.
The Tab completion also works for a nickname in a query window, the nicknames in your notify list when in the status window, and for channel names in your favorites folder when in any window.
You can also use wildcard *? characters in nickname tab completion.
