Why cant I get a boot menu display on extrnal display on headless mac?

MacBooks typically don’t show the boot menu (Option key) on external displays during startup because the external GPU output initializes only after macOS loads. Since your internal screen is broken, you can't see the boot picker. This is a hardware limitation. A workaround is using screen sharing, target disk mode, or replacing the internal display to view the boot menu.

Within the display lid is a magnet which mates up with a Hall sensor to identify when the display is down. Most of the time this is to save power by disabling the display power. It is also used to identify the primary display. By default it's the internal, but when the external is desired the closed lid tells the logic to pump the video to the external. We refer this as Clamshell Mode.

But here we have no lid to tell the logic to pump the video to the external so none of the first phases of system booting up are seen. You will need to fake the system out by placing a rare earth magnet on the uppercase where the lids magnet would mate with the Hall sensor.

To help locate the sensor here's an image on it

So placing the magnet about the same distance forward along the palm rest of the distance between the USB-C ports should hit it.

Hey, I’ve been through something similar, so I get how annoying this can be. Unfortunately, older MacBooks (especially Intel-based ones) don’t always mirror the boot menu (like when you hold the Option key) to an external display right away — the built-in screen usually has to initialize first, even if it's broken. That’s likely why you’re not seeing anything during startup, even though it works fine once macOS loads.

One workaround some people try is closing the lid after the chime while the Option key is still held — this sometimes forces the Mac to use only the external display (called "clamshell mode"). Not a guarantee, but worth a shot.

Another approach: You could try setting the USB stick as the startup disk from within macOS before rebooting. Go to System Preferences > Startup Disk, select the Linux USB, and restart — that way, it skips the Option menu.

But yeah, sadly, Macs aren't always great about headless setups during boot. Hope one of those tips helps!