This can only be done currently in /boot/dietpi.txt, and, must be set on a fresh install, prior to 1st boot:
#Global Password to be applied for the system
# Affects root login password, and, all software installed by dietpi-software, that requires a password
# eg: MySQL, Transmission, Deluge etc.
# WARN: Passwords with the any of the following characters are not supported: \"$
# WARN: Do NOT change this entry, after 1st run setup of DietPi has been completed. It is always scraped by dietpi-software.
AUTO_Global_Password=dietpi
And:
#Optional username for nextcloud admin account, default is admin. Applied during installation.
dietpi_nextcloud_username=admin
There is a bug in v140 that causes a failed NextCloud install if using global password above. This is fixed for v141, but v141 is not completed yet, so you’ll need to either wait, or, use the testing branch: DietPi/BRANCH_SYSTEM.md at master · Fourdee/DietPi · GitHub