https://forums.rancher.com/t/longhorn-ui-with-traefik/16742
I managed it to route the longhorn-ui via my traefik. So I can answer my question by myself. Here is my receipt:
1.) first remove the default service provided by Longhorn deployment:
kubectl delete service longhorn-frontend -n longhorn-system
2.) next create a service definition (longhorn-service.yaml)
apiVersion: v1