Evidently your SETUP.EXE does not give focus to the edit box you want when it starts, so I think all you need to do is TAB your way to the desired edit box before sending your text.
in reply to Help needed for using Win32::Setupsup module
I have had to do this for a Win32 GUI app which didn't include all the edit boxes in the TAB sequence, and in that case I was able to use Win32::GuiTest to move the mouse pointer into the edit box, sent a mouse left click and then send the text - horrible in concept but worked perfectly in practice. You need to watch for screen resolution differences and calculate the mouse coordinates on the fly, but it can be done if the window always appears at the same position.
I'd like to be able to assign to an luser