How to get back a device you hit "Forget this device" on iphone 8

You will have to put the device back into discovery/pairing mode. For the car, follow the instructions in the owner's manual for the car. Generally you just go to the Settings for the phone in the car and select pair. After doing that with the car, then turn on Bluetooth on the iPhone, and follow the on-screen instructions in the car.

If you connect using Bluetooth

1. Restart your device.
2. Go to Settings > Bluetooth, and turn off Bluetooth. Wait for 5 seconds, then turn Bluetooth back on.
3. Check your car stereo's user manual for how to pair with a Bluetooth device. Most cars require a phone setup on the car display. 
4. If you've connected multiple phones to your car stereo, try renaming your device: Go to Settings > General > About > Name, and type a new name. Then try connecting again.
5. Update your device.
6. Install any firmware updates for your car stereo.
7. If you still can't connect, contact Apple Support.

If you connect using USB playback

1. Restart your device and your car.
2. Make sure that your device is unlocked and on the Home screen.
3. If possible, try to connect using another cable or USB port. Check your car owner's manual to learn how to connect an Apple device.
4. Check your charging port for any debris or damage. Make sure that your charging cable is inserted fully into the port.
5. Update your device.
6. Install any firmware updates for your car stereo.
7. Try playing audio from a different app. If you can't play audio from a third-party app, contact the app developer for help. If you can't play audio from an Apple app, contact Apple Support.

Information about products not manufactured by Apple, or independent websites not controlled or tested by Apple, is provided without recommendation or endorsement. Apple assumes no responsibility with regard to the selection, performance, or use of third-party websites or products. Apple makes no representations regarding third-party website accuracy or reliability. Risks are inherent in the use of the Internet. Contact the vendor for additional information. Other company and product names may be trademarks of their respective owners.

If your iPhone, iPad, or iPod doesn't connect to or work in your car ...

https://support.apple.com/en-us/HT203412