Live CD user change?

Question

Kubuntu

I can't find information on how to change default user in kde neon in live CD
in what files default user can be registered.
tried to create a user and so sudo useradd pupkin when the system boots, a black screen
and specifying the shell sudo useradd -m -s /bin/bash pupkin does not
help . /casper edit/usr/share/initramfs-tools/scripts/casper-bottom/10adduser there isn't really anything there either.