The Ibus plugs into the USB port on the back of the radio. You connect the red wire to switched 12v. The brown goes to ground & the white goes to the Ibus. The green you won’t be using. Also see instructions here.
IBUS Interface for BMW cars
I used the the “other” amp or antenna power turn on source to power the Ibus device, I used the ground to the radio & I used the now defunct radio Ibus wire (it’s colored exactly as described in the link).