The info is available below (serviceIpAddress). So you could use [reference()] to refer to that.
GET /subscriptions/.../resourceGroups/.../providers/Microsoft.Web/hostingEnvironments/.../capacities/virtualip?api-version=2018-11-01
{
"serviceIpAddress": "...",
"internalIpAddress": "...",
"outboundIpAddresses": [
"..."
],
"vipMappings": []
}
Suwatch