MacBook USB-C Video Out

Many video adapters and multiport hubs will not support video out until MacOS has booted to a login screen.

Apple's own USB Type-C Digital AV Multiport Adapter does not display video out until macOS is booted.

The most reliable means of getting boot-menu and Apple-logo loading screen output is with the internal display connector fully disconnected.

Most USB-C to Displayport adapters should support boot menu display out natively when the internal display is disconnected, because there's no intermediary adapter between the USB-C port's native DisplayPort Out and the monitor itself. This USB-C to DisplayPort cable is confirmed as working at the boot menu with a T2-era MacBook Pro A2159, among other later-era MacBooks equipped with USB-C / TB3 / TB4. It is a DisplayPort 1.4 adapter so it supports 8K/60Hz and lower resolutions at higher refresh rates.

Many HDMI adapters will not work reliably as the USB-C port does not put out a native HDMI signal, but this adapter is tested and works to display the EFI portion of boot so you can see the Apple symbol and the FileVault screen. It is a HDMI 2.0 adapter so it also supports 4K/60Hz display.