The superuser link are just workarounds to check either the day of week or the “first N-day of month” condition in the executed code itself.
Of course you can do that in TrueNAS - that needs to go into the command field. Like with every cron implementation.
Example: (TN CORE)
Replace the echo "It's the first Saturday!" part with the real command you want to run.
HTH,
Patrick