{
  ...
  "resources": [
    {
      "apiVersion": "2015-08-01",
      "name": "[concat(parameters('websiteName'),'/',parameters('websiteCustomDomainName'))]",
      "type": "Microsoft.Web/sites/hostNameBindings",
      "location": "[resourceGroup().location]",
      "properties": {
        "domainId": null,
        "hostNameType": "Verified",
        "siteName": "parameters('websiteName')"
      }
    }
  ]
  ...
}