In ‘external’, you put ‘false’ for the first app participating in this network, which will create it.
For subsequent apps that will participate in the same network, you put ‘true’, and they won’t try to create the network, they will only join the defined one.
And in the service, you add the tag where the app will be:
networks: - jellyfin
The only thing you have to worry about is that the app that creates the network has to be the first one created.
I haven’t been able to solve the stack name change yet; if anyone has any suggestions, but I think that can only be done by editing the internal controls of TrueNAS.