First of all, we recommend using a still camera for the highest quality. And when we support a camera, generally that means it provides live view over USB, not HDMI. See our Camera Support page for a list of supported cameras and instructions to configure them to work with Dragonframe.
In some cases, though, it may be useful or necessary to use the HDMI output of a camera, either as just the video assist source, or also as the capture source.
In those cases, you will need to use an HDMI capture device to get the HDMI signal converted to a usable video source. You cannot plug into the computer's HDMI port, because that is an output for a monitor.
The following devices should work. However, please make sure you pick a device that is compatible with your computer, your operating system, and your camera's HDMI output:
Any of the Blackmagic Design devices listed on our Camera Support page.
Elgato Cam Link 4K
Atomos Connect 4K