USE "IN TRANSIT" CUE

Use the “in transit” cue. Ask Siri to help you find an address with a couple extra words at the end, as in: “give me directions to San Francisco International Airport via transit.” iOS will open up the Maps app and display the routing pane, which lets you choose which third-party app you’d like to supply your directions.
Attentive readers will know that Google Maps is included among these routing apps, which means that you can actually use this trick to get directions via Google Maps using Siri.