The game supports no native widescreen. There are two workarounds, one better than the other.
Alternatively you can:
WindowWidth = *your window width*,
WindowHeight = *your window height*,
This fix however produces a series of graphical errors.
As of July 2019 latest steam version of the game includes native 1920x1080 resolution support via in-game options (It's also highly possible that higher resolutions are also available)