Depending on the game platform you are playing on will determine the button configuration required to perform the crouch ability. Here is a quick guide on how to do it on the various platforms.

Press Left CTRL
On PS4
Standard Layout: L1
Quick Builder: L1
Combat Pro: Right Thumb Stick, Tap to crouch)
Xbox
Standard Layout: LB
Quick Builder: LB
Combat Pro: Right Thumb Stick, Tap to crouch)