use a Raspberry Pi 400 as a normal USB keyboard via USB-C
- download https://github.com/No0ne/keyboard400/archive/main.zip
- extract all files from the zip archive onto a micro SD with FAT32 format
- insert the micro SD into the Pi 400 and plug it into another computer using a USB-C to USB-C or USB-C to USB-A cable
kernel, modules and firmware files copied from: https://github.com/raspberrypi/firmware
busybox binary copied from: https://packages.debian.org/sid/arm64/busybox-static/download