When you can't easily port forward to a service, here's how to grab a single pod from the matching selector and port forward to it.
NAMESPACE=
INSTANCE=myapp
CONTEXT=
PORT_LOCAL=
PORT_REMOTE=
pod=$(kubectl get pod --namespace $NAMESPACE --selector "app.kubernetes.io/instance=$INSTANCE" --template '{{ (index .items 0).metadata.name}}')
kubectl port-forward --context $CONTEXT --namespace $NAMESPACE --address 0.0.0.0 "pod/$pod" $PORT_LOCAL:$PORT_REMOTE