It's not really a code though.
Admin > Current Theme > Reset Options
Click "Reset default (guest) options for this theme"
Where it says "Use quick reply on topic display choose: "show, on by default" then Save
Click "Remove all members' options and use the defaults"
click Ok
If you don't want to reset them all they can choose to use it or not themselves.
Profile > Look and Layout Preferences > Use quick reply on topic display:
