The game only let's you choose from set refresh rate options (30, 60, 90, 120, 144). If you want to set the game to any other refresh rate you have to use the console:

1. Press F1 to open the console
2. Use the command: 'framerate 240' (or any other desired refresh rate)
3. Restart the game to reenable achievements

I hope that this helps some fellow 240 hz gamers out there!