IIRC you have to apply for it first on the website to show your interest.
Assuming that's done, the setting to turn it on's a little buried. Open the app. Tap the bullet points in the lower right and then the slide-button thingies on the lower right on the "Trip Planner" screen (what this has to do with planning trips I have no idea).
This should get you to Driving Preferences; here you can enable/disable "Deliveries". Hope that helps!