- Clone this to your Steam's compatibilitytools.d folder, which is
probably
~/.steam/steam/compatibilitytools.d/.
- Restart Steam.
- Right-click Morrowind in your library.
- Go to Properties, then Compatibility.
- Check the checkbox to force the use of a specific Steam Play
compatibility tool.
- Select OpenMW Wrapper from the dropdown, and you're done. Clicking
Play on Morrowind should launch OpenMW, and Steam should recognize
it running.
Note that this only works if you have OpenMW installed and set up, so
that openmw launches OpenMW.