Source: Moeomu’s blog

The magnet trick

Find two magnets and place them near the audio of the MacBook to trick the system into closing the lid to turn off the built-in screen.

NVRAM boot setup method

This method enables the screen to be displayed separately on the external screen from the driver level.

Enable

• Reboot, press and hold Command+R key
• Enter the password to boot setup - terminal
• Enter the command nvram boot-args="niog=1"
• Connect the external monitor, connect the power supply and reboot
• Close the cover immediately after booting by entering the user password, then see the external screen with a screen, open the cover of the MacBook, success

Restore

• Reboot, press and hold Command+R key
• Enter the password to boot setup - terminal
• Enter the command nvram -d boot-args
• Reboot