It will be controlled by Truenas not Proxmox. Truenas can add swap space to each drive automatically: www.ixsystems.com/documentation/…/storage.html

But you probably already have existing drives so that doesn’t help. This might though: wiki.debian.org/Swap

But be aware that Truenas is design to be an appliance and doesn’t really want you tinkering under the hood. So you may have to manually add the SWAP after each boot of TN.

I would guess the best long term fix would be moving services out of the TN VM and into a different VM.