How to set default root password for mobile-nixos

Hello, I would like to try mobile-nixos on my Acer Spin 513 laptop. I made a USB boot flash disk.
I followed this page: Acer Chromebook Spin 513 — Mobile NixOS
Made this command nix-build --argstr device acer-lazor -A outputs.kpart and then this:
nix-build --argstr device acer-lazor -A outputs.default
Then I flashed the generated image to flashdisk and booted in my chromebook.
It boots, I saw the mobile-nixos logo but I dont know the name of default user and password.
I did not set anything, just ran those 2 commands to generate an image.
So please, what is the default root password or how can I set my own password before I can login?

Same issue. I mounted the image and delete password in /etc/shadow needed for root user. Then you can directly login.