Yes, the app works great !
You open the app, find your place on the map ( your cell phone is tracking you, so you’ll be near the nearest dock.
I have taken a screenshot, assume I have chosen the station near Bryant Park.
You click on the station, and then click “unlock a bike” at the bottom left corner. Then you’ll get a five digit code consisting of the 1, 2, & 3 buttons on the dock. You have thirty seconds to press the buttons.

CitiBike App Station at Bryant Park (example).