Thank you for being so nice with me. I will type EXACTLY what you said the first time into status window:
/window -a @targetwindow
/editbox -n @targetwindow texthere
And what do I get: a custom window @targetwindow without an editbox and
-
* /editbox: invalid window
-

Now, that's all very fancy and stuff, but there's no editbox to use the up arrow to see if there's some history in it, and there's no extra history in the status window editbox either.
So I'm kinda thinking 'maybe he overlooked something small' and at least put in an actual editbox so there's no error anymore. This results in absolutely nothing for that custom window. Nothing is done (that's ok for me) but when I use the up arrow in the editbox, there's still no history, only the aint-nothing-here-beep.

----

Now for the script stuff, after carefully reading it and testing it out, it appears the /editbox command only works ok when the @win/#chan is active. I have the habit of typing those commands in Status Window, so then it won't work. Also you have to use the on INPUT trigger to halt the text. It's a hack, but apparently there's no other way...

Here's what the example above should have looked like:
//window -ae @targetwindow | editbox -n @targetwindow texthere
//window -a #channel | editbox -n #channel texthere
warning: use on *:INPUT:@targetwindow,#channel:halt to prevent it from executing/sending it

Anyway, your script seems to do the trick, so irssi-lovers have another reason to use mIRC smile