Well you can but you have to setup an on input...
The on INPUT event triggers when you enter text in an editbox and press enter.
Format: on <level>:INPUT:<*#?=!@>:<commands>
Example: on 1:INPUT:#mIRC:/echo You entered the text " $1- " in the #mIRC window
on 1:INPUT:#:/echo I just mumbled " $1- " in a channel
Triggers when you enter text in an editbox in a channel window and press enter. The $1- parameters refer to the text that you entered. If you /halt this event, you can prevent mIRC itself from processing your message.
on 1:INPUT:?:/echo I just mumbled " $1- " in a query window
on 1:INPUT:=:/echo I just mumbled " $1- " in a dcc chat
on 1:INPUT:!:/echo I just mumbled " $1- " in a fileserver
You can also specify a specific channel/window name instead of *#?=!@.
You can use the $ctrlenter identifier to test whether Control+Enter was pressed when the user entered the text.
Note: You can use commands such as /say with on INPUT and they will send the message to the window in which you are typing, however most commands/events do not work this way and require you to specify the actual destination of a message.