The easiest way to set it up is to run nwn first.
Run nwn - and when nwmain.exe is running, alt + tab to desktop.
In OBS - in your scene editor, add a game capture element, and set the process / game to nwmain or neverwinter nights - whichever it is named.
When you start broadcasting - it will then stream a black screen, until it detects that game appearing.
It should automatically switch over when you start playing your game.