Step by steyp to install ZeroTier on TrueNAS Scale.
Latest version TrueNAS Scale ElectricEel 24.10:

Add ZeroTier Network ID; enable Host Network; leave Auth Token, Identity Public and Identity Secret empty:

On ZeroTier console, authorise TrueNAS access the network:

ZeroTier is happily running in TrueNAS:

References
- How to install zerotier on TrueNas Scale? https://www.reddit.com/r/truenas/comments/1dvdp36/how_to_install_zerotier_on_truenas_scale/