You need to login into Truenas, go to storage -> Manage Devices -> select your pool and then the first drive. On the right side on the Disk Info part press "Edit" and there you have the possibility to set the HDD Standby and/or Advanced Power Management. Under Advanced Power Management everything from Level 1 to 127 will spin down the drive, from 128 to 254 will not spin down the drive. You have to do this for every HDD.
I don't know how Hexos is reacting to this and which side effect it can have, also for other people reading this, spinning down drives is generally not recommended because the spin ups are putting wear on the drives, and drives are vulnerable during spin ups. When drives are spinning there is an air cushion which is keeping the head of the disks, while this is not the case during a spin up and a head crash can occur. Also when leaving a drive sitting without spinning it can have a negative impact on the lubrication of the bearing. Also make sure that the drive doesn't not cool down to much, this is especially import in the winter if the server is sitting in a cellar or basement where the air temperature can drop and this can also cool down your drive significantly, depending on your setup. There have been plenty of perfectly working drives without issues, which did not recover from a spin up. So be warned, that it might shorten the lifespan of a drive.
Also you might want to look into enabling the different C-States in the Bios, this can also save a significant amount of energy.