For Longhorn v0.8.0+, UI service type has been changed from LoadBalancer to ClusterIP

You can run kubectl -n longhorn-system get svc to get Longhorn UI service:

NAME                TYPE           CLUSTER-IP      EXTERNAL-IP      PORT(S)        AGE
longhorn-backend    ClusterIP      10.20.248.250   <none>           9500/TCP       58m
longhorn-frontend   ClusterIP      10.20.245.110   <none>           80/TCP         58m

To access Longhorn UI when installed from YAML manifest, you need to create an ingress controller.

See more about how to create an Nginx ingress controller with basic authentication here