No app and no sign-up. Your ticket opens in your phone's browser. Get it in any of these ways:
At drop-off
The attendant shows a QR code on their phone. Point your camera at it to open your ticket.
By text
If you gave your mobile number, tap the link in the text message.
At the valet stand
Scan the QR sign on the podium and type your license plate.
From your ticket you can see your car's status, tap "Request My Car" when you're leaving, and tip your valet by card or Apple Pay.