If the webcam is itself powered via USB, the iPad alone may be unable to provide sufficient power. In such circumstances you'll need to use an Adapter that has both a USB connection for the camera and a USB-C pass-through power connection.
- USB-C Digital AV Multiport Adapter
https://store.apple.com/xc/product/MW5M3ZM/A
Your iPad also supports connection of a powered USB-C hub - which can provide power to both your iPad and any connected USB accessories.

In the above image, you'll see that this Adapter has three ports. From left to right:
- USB-C port for connection to your Power Adapter
- HDMI port to connect to a monitor or TV if required
- a standard USB Type-A port to connect USB devices - such as a webcam or USB storage device
You may find this article from Cult of Mac helpful:
https://www.cultofmac.com/how-to/how-and-why-to-use-an-external-webcam-with-your-ipad