In PolypopLive, the "Webcam" source captures video from a device, whether it's a physical hardware device or a virtual one. To add a capture card as a source, you'll need to add a Webcam source to your Library panel and then select your capture card in the webcam's "Device" layer properties.

To learn how to add a webcam source to your Library panel, please review the “Adding your Webcam” document.