Displaying static video content on the TwinCAT HMI is possible by using a preconfigured Video user control. Also, displaying conent from a stream is in some ways possible with an iFrame. But directly showing usb webcam data is currently not available.
This repository holds a Desktop.view example that adds a video control to the main view and uses navigator.mediaDevices.getUserMedia to search for a connected webcam to display in the video control.
Just add the provided Desktop.view file to a new HMI project and make sure the .view is set as the startup file. Opening the live view should now be able to show the connected camera.
The TwinCAT HMI engineering server is always running in the background, so it can happen that the webcam access is locked by this process. This means that when you open the live view in a browser no video is shown.
If no picture is shown anywhere, check the browser console. The error message is logged there and the documentation page can help you find the error.