Update tutorial-ingress-controller-add-on-existing.md

Author: JackStromberg

articles/application-gateway/tutorial-ingress-controller-add-on-existing.md

@@ -54,6 +54,9 @@ To configure more parameters for the above command, see [az aks create](/cli/azu
 > [!NOTE]
 > A node resource group will be created with the name **MC_resource-group-name_cluster-name_location**.
 
+>[!NOTE]
+>If you are planning on using AGIC with an AKS cluster using CNI Overlay, specify the parameter `--aks-custom-headers AKSHTTPCustomFeatures=Microsoft.ContainerService/AppGatewayWithOverlayPreview` to configure AGIC to handle connectivity to the CNI Overlay enabled cluster.
+
 ## Deploy a new application gateway 
 
 You'll now deploy a new application gateway, to simulate having an existing application gateway that you want to use to load balance traffic to your AKS cluster, **myCluster**. The name of the application gateway will be **myApplicationGateway**, but you'll need to first create a public IP resource, named **myPublicIp**, and a new virtual network called **myVnet** with address space 10.0.0.0/16, and a subnet with address space 10.0.0.0/24 called **mySubnet**, and deploy your application gateway in **mySubnet** using **myPublicIp**.