Download the latest version of niohresolution.exe from here: https://github.com/LennardF1989/NiohResolution/releases

Place that file in the nioh.exe folder, located in ~\Steam\steamapps\common\Nioh

Run the niohresolution.exe

Launch the game, and in the launcher settings, change quality to high and resolution to 1920x1080 or 2560x1440 and save the settings

Now when you click on launch game, the UltraWide fix should be enabled.

I did not create this, all credit goes to https://github.com/LennardF1989 and jackfuste from wsgf forums.