If you’re using one of the “community apps”, and not a custom YAML file, you cannot do this. TrueNAS does not support VPN, nor do the community apps.
Here’s a post I wrote a while ago showing how to do this for Qbittorrent. There’s some similar posts in that thread and others. Basically, you have to install “Dockge” from the community apps, then inside that set up docker-compose YAML files for “gluetun” (to manage the VPN) and your app (Tachidesk in this case). You can also put both of them into a single YAML file if you wish, but I prefer keeping them separate in case I want other apps to use the same VPN.