I was at a dealership with a 2017 Bosch Performance Cx that was being updated to enable the Walk Assist mode.
It was a cool feature, particularly on this large fat bike that weighed 65 lb easily.
When my 2016 Performance Line Speed was updated, it showed no Walk Assist mode to enable.
It's a firmware issue that depends on whether the feature exists on your bike, if it does, it's just a software adjustment to get it going.
Once enabled, the throttle is controlled using the + button on your boost level controller after you click the small button on the upper side of the controller.
If you use it and stop, you have to activate it again with that small button. It's a single use feature from my experience.