Route request to internal loadbalancers like nginx

Networking
1

I have istio ingress gateway installed and also configured virtualservices. However I have a special case that few services are loadbalanced via an internal loadbalancer with nginx. I would like to route the requests from the Istio Gatway to the nginx loadbalancer. I tried by rewriting the url but it does not work since I do not have a route destination. Also I tried redirect but it does more of a browser redirect. Can you please let me know how can I achive routing calls to the nginx internal loadbalancer.

Sample ingress

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  annotations:
    kubernetes.io/ingress.class: nginx
    nginx.ingress.kubernetes.io/rewrite-target: /$1
  name: dev-ingress
  namespace: dev
  labels:
    app: dev-ingress
spec:
  rules:
  - http:
      paths:
      - path: "/test-nginx"
        backend:
          serviceName: test
          servicePort: 8050

Sample VirtualService.

      headers:
        request:
          add:
            X-Forwarded-Prefix: /test/
      match:
      - headers:
          x-original-host:
            exact: test.com:15540
        port: 15540
        uri:
          prefix: /test/
      redirect:
        uri: /test-nginx