It seems that it's not possible to set focus to the editbox without affecting its content. For example, I want when I activate a window to automatically set focus to the editbox while leaving whatever is already there intact. So I think there should be a new switch for that or modify the behaviour of -f, so that when no new text is specified, it only sets focus to the editbox (and not clear the text).
/editbox -f <window> $editbox(<window>) is not good enough: by overwriting the editbox text, it resets the cursor position (which I find annoying) and there's always the consecutive-spaces issue (/editbox would kill any existing double spaces in the editbox).