Hook your Pi 400 up to your PC somehow, using a USB Type-C cable into the power port. Anker make good ones- I used a 3m white one for my tests.

Our USB-C to USB-A is great if you're using a USB-A port (but make sure it's a high power one): https://shop.pimoroni.com/products/usb-c-to-usb-a-cable-1m-black

This project started out life as a gist - https://gist.github.com/Gadgetoid/5a8ceb714de8e630059d30612503653f

Thank you to all the people who dropped by with kind words, suggestions and improvements.

Add dtoverlay=dwc2 to /boot/config.txt

Reboot!

sudo modprobe libcomposite

wget https://github.com/Gadgetoid/pi400kb/blob/main/pi400kb?raw=true

chmod +x pi400kb

sudo ./pi400kb

YOUR PI 400 IS NOW A FREAKING KEYBOARD FOR YOUR PC WHAAAAT!?

Your keyboard input will be detached from your Pi while it's forwarded to your host computer.

Press Ctrl + Raspberry to exit and restore your keyboard on the Pi.

sudo apt install libconfig-dev
git clone https://github.com/Gadgetoid/pi400kb
cd pi400kb
git submodule update --init
mkdir build
cd build
cmake ..
make