[tmzt]

I'm not sure if this would work for you, but there are inexpensive devices that plug into an HDMI port. They appear to the computer as a monitor. I use them for screen sharing to a remote display, but they should enable to think there is a monitor attached. It negotiates the display information as if it was an actual monitor.

Here's the pack of three I purchased on Amazon.

Woieyeks 3 Pack HDMI Dummy Plug https://www.amazon.com/dp/B0CKKLTWMN