To make crouch momentary (instead of toggle)

Within WillowInput.ini:

Search for the following line under [WillowGame.WillowPlayerInput]:

advancedbutton bAdvancedButtonAux1 | SwitchSeats

and change to

Duck | SwitchSeats

You can remove the SwitchSeats variable if you do not wish to have the key switch seats as well.

If this doesn't work, make sure this line exists under [WillowGame.WillowPlayerInput], otherwise add it:

Bindings=(Name="Duck",Command="Button bDuck | Axis aUp Speed=-1.0 AbsoluteAxis=100",Control=False,Shift=False,Alt=Fa lse,LeftTrigger=False,RightTrigger=False,bIgnoreCt rl=False,bIgnoreShift=False,bIgnoreAlt=False)