Select the panel view of the panel that's down on the left. You can see it from the drivers seat but only can operate it from the camera view. There is a switch to activate the AFB. If you have a keyboard it's shift + r to activate it.
Once you enabled the switch on the panel down on the left (the switch is on the upper left of the panel if i remember right) set the loco up as usual. Releasing parking and nomal brakes and all that stuff. Once fully released, set the AFB lever to the speed you dont want to exceed. After that apply power with the normal Power lever. The Train should accelerate to the set speed and not go over it
In order to activate AFB while you are moving you need to move the the AFB to the maximum speed first. Then turn on the AFB on and move the lever back to the speed you do not want to exceed.
I have a further question on this on the 185.2. As I was coming towards my stop I wanted to take off AFB and go back to manual control. What I did was switch the AFB lever to 0, turned off the AFB switch. Then I dropped the throttle to 0 then increased it again thinking it would some sort of reset and I would get manual control. Instead I cane to a full stop! I know this is def wrong, what did I miss? AFB is great though in general