You need an HDMI to USB adapter or bridge. A bridge will include connections for external microphones, and adapter would just have HDMI.
Extron MediaPort 200
Vaddio AV Bridge
Both of these use UVC drivers, so you don't have to install any software to use them. Avoid anything that requires proprietary drivers or software.
Adapters are less expensive, but I can't recommend one specifically. Here's an example: