On my Debian 12 based home server I wanted to set the CPU frequency scaling governor to "powersave" upon boot. The cpupower package does not have a default configuration that is applied on startup, so I had to do this myself.
With systemd 252.17-1~deb12u1 I had to do the following:
/etc/systemd/system/cpupower-config.service
[Unit] Description=Configure kernel to use "powersave" cpu scaling governor [Service] Type=oneshot ExecStart=cpupower frequency-set -g powersave [Install] WantedBy=multi-user.target
Then I only had to enable it:
$ systemctl enable cpupower-config.service