Script call to change message window opacity

Hi all,
I know how to change the opacity in general (@ window_base). But I don't want the opacity to change in the whole gane. I just want the windowset to be invisible when I'm talking to someone.Afterwards the opacity should return to normal. Is there any way to do this?
 

invisible? oh, that's easy, when you use the message box command you can sei it to normal, dark or invisible through a drop-down menu there.