I’m not sure when they received the test drive demo Gravity, but I scheduled my test drive with my SA at Tysons last week. I’m going in this weekend to do a longer test drive. I think you can call the studio or service center to schedule the test drive. I’m not sure if it is restricted to customers with pending orders.
As pointed out elsewhere, just tap the button as if you do want to schedule a test drive, but then don’t actually finish the process. The app will stop asking after that.