Get BootScreen on external monitor

I have a MacBook Logic Board I would like to turn into a small desktop computer. Although I cant get the boot screen to show up on the external display via Mini Display Port. How can I get it to treat the external display as the native one? I know normally with the lid shut the computer should automatically use the external display, but since the lid is not connected it doesn’t do this. I’d be happy to jump a connector on the logic board if I knew which one it was but I believe the connector is part of the main LVDS cable and the contacts on that are way too small for me to mess with.

Any advice would be appreciated.