How to use 3rd party routing instead of Apple Maps?

[rocknblogger]

I have seen a few comments and screenshots like this one giving you options to get routes using 3rd party apps like Navigon, TomTom and now Google Maps from within Apple Maps. Even the updated TomTom app tells you that you can do this (Though does not explain how). Also here's a Macrumors post that shows a screenshot (The first screenshot) with the option.

For the life of me I cannot figure out how to do this

 

[Small White Car]

When you go to get directions, pick the bus instead of the car and hit 'Route.'

 

[rocknblogger]

When you go to get directions, pick the bus instead of the car and hit 'Route.'

Awesome!! Thank you. That could not be hidden any better
I guess I didn't try everything