HTTP Stauts code 500 for an IIS based application.

There's usually one of two reasons this sort of thing will happen:

1. A rewrite rule isn't catching all of the :8090 URL references sent to the client

2. The server actually requires this :8090 specified in the Host header

That's not to say something else could be wrong here, but given that you're getting a 500 response from the server, that leads me to believe you're asking it a question it doesn't like. Is it at all possible that the application, or portions of it, requires the port designation in the Host header? Try this:

when HTTP_REQUEST {
    if { not ( [HTTP::header Host] ends_with ":8090" ) } {
        HTTP::header replace Host "[HTTP::header Host]:8090"
    }
}

This will add the :8090 to the Host header on ingress. If that doesn't work and you're still getting 500 responses, you may have to take a closer look at the requests and compare direct vs. proxied.