Setup a router in toml on docker-swarm

When I declare a router in the dynamic conf, how does traefik know on which container to set it? I don't get it from the docs...just worked with labels for now:

https://docs.traefik.io/routing/routers/#domains

I need to declare it in toml, because I wan't to run a mailu.io stack...the container won't start without certificates, and traefik won't recieve the labels if the container doesn't start -> ergo the container never starts...