https://www.raspberrypi.com/news/raspberry-pi-bullseye-update-april-2022/

To set up a user on first boot and bypass the wizard completely, create a file called userconf or userconf.txt in the boot partition of the SD card; this is the part of the SD card which can be seen when it is mounted in a Windows or MacOS computer.

This file should contain a single line of text, consisting of username:encrypted- password – so your desired username, followed immediately by a colon, followed immediately by an encrypted representation of the password you want to use.

To generate the encrypted password, the easiest way is to use OpenSSL on a Raspberry Pi that is already running – open a terminal window and enter

echo 'mypassword' | openssl passwd -6 -stdin

This will produce what looks like a string of random characters, which is actually an encrypted version of the supplied password.