SOAP requests must use /soap/FlightXML2/op as the endpoint, and the request must be encoded as an XML document in the standard SOAP request body format.
I recommending using the JSON interface to FlightXML instead, if you are needing to generate and parse the bodies manually.
Save the following file as test.asp, but substitute your actual username and API key:
Dim postData, httpRequest, jsonResponse
postData = "airport=KAUS"
postData = postData & "&howMany=10"
postData = postData & "&filter=airline"
Set httpRequest = Server.CreateObject("MSXML2.ServerXMLHTTP")
httpRequest.Open "POST", "http://flightxml.flightaware.com/json/FlightXML2/Arrived", False, "YOUR_USERNAME", "YOUR_APIKEY"
httpRequest.SetRequestHeader "Content-Type", "application/x-www-form-urlencoded"
If httpRequest.status = 200 Then
Set jsonResponse = JSON.parse(httpRequest.responseText)
For Each flight In jsonResponse.ArrivedResult.arrivals
Response.Write("Flight " & flight.ident & " is arriving from " & flight.origin & "<br>")
Set httpRequest = Nothing