Port Forwarding by Autostart

What is the easiest way to start this script (port forwarding) on DietPi by Autostart ?

ssh -fNT -p 22 \
    	-L 10.144.xx.xx:yyy: \
	-L 10.144.xx.xx:yyyy:

There used to be /etc/rc.local for that, but not anymore. You can try to add a custom cronjob in /etc/cron.d/ with @reboot. Keep in mind that you also need to specify the user who will run the command.
Alternatively, look at dietpi-autostart option 14.

Or use a systemd service to start on boot. Or DietPi custom script available as autostart option.

Easiest is probably to put the script into /var/lib/dietpi/postboot.d. All contained scripts are executed as root user independant of a login once at the end of the boot sequence.