For people who prefer not to use a HDMI dummy (or physically cannot put one in), a workaround is to use xserver-xorg-video-dummy

It creates a dummy device, monitor and screen running in your host so RustDesk can connect to the display created.

sudo apt-get update
sudo apt-get install xserver-xorg-video-dummy

sudo nano /etc/X11/xorg.conf.d/10-dummy.conf

This is an example configuration file.

Section "Device"
  Identifier "Dummy Device"
  Driver "dummy"
  VideoRam 256000
EndSection

Section "Monitor"
  Identifier "Dummy Monitor"
  HorizSync 28.0-80.0
  VertRefresh 48.0-75.0
  Modeline "1920x1080"  172.80  1920 2040 2248 2576  1080 1081 1084 1118 -hsync +vsync
EndSection

Section "Screen"
  Identifier "Dummy Screen"
  Monitor "Dummy Monitor"
  Device "Dummy Device"
  DefaultDepth 24
  SubSection "Display"
    Depth 24
    Modes "1920x1080"
  EndSubSection
EndSection

Section "ServerLayout"
  Identifier "Dummy Layout"
  Screen "Dummy Screen"
EndSection

sudo reboot