Pandora does not require or generate an activation code when connecting to your vehicle with Bluetooth. Those types of codes are used when connecting Pandora to an in-home device such as a TV or smart speaker.
If your iPhone is already connected to your vehicle via Bluetooth, then you'll just need to launch the Pandora app on your phone to start listening.
If you're having trouble connecting your iPhone to your vehicle with Bluetooth, I'd refer you to the car's manual for specific steps connecting your phone to the car's system.
Use your mobile device only when conditions allow and as permitted by applicable law.