The default behavior of World of Warcraft when pressing the / key is to open a chat box and type / into it. This means in order to type: /BigInputBox, you would not need to press ENTER first. You would simply need to press / followed by BigInputBox. This should likely be included as those who are used to starting slash commands via / instead of ENTER will often by instinct press / expecting for a chat box to open.

Can't do it. We can't put text of any sort into the inputbox without removing its secure status, and thus breaking its ability to cast spells and commands. This is why I've chosen to not redirect that keybind to our box. This is a blizzard limitation.