How can I charge my iPad Pro in my car’s USB port? My iPad has a USB C connection.

You will need an appropriate cable - and ensure that the USB port can provide adequate power. The USB port in your car must be able to supply at least 2.0A at 5V (10W) - to charge your iPad. Charging will be slow.

If your car has a regular 12V outlet, you may achieve significantly better charging if you purchase a USB-C PD (Power Delivery) auto-charger - and connect your iPad via a USB-C to USB-C cable. This has the capability, subject to the charger rating, of charging your iPad Pro at its maximum supported charging rate - 18W (2A @9V / 1.5A @12V).

Here is one example marketed by Apple - manufactured by Anker:

• mophie Dual (USB-C/USB-A) Car Charger

https://store.apple.com/uk/xc/product/HN822ZM/A

Aside from the cable issue I would doubt that your car's USB port would charge your iPad as those ports are usually just for auxiliary playback purposes and not charging. Using a 12V adapter that plugs into the 'cigarette lighter' receptacle will be the better choice. Back to the cable which you would still need if you use a 12V charger, Amazon sells the cable you are looking for (USB-A to USB-C charge cable). Hope this helps.