Hello There,
If the proxy is set for explicit forward, and you set the browser for http traffic to the port configured on the ironport, (browser explicitly configured proxy settings with ironport ip port 80 or 8080), and if the application is http, and uses the browser settings for proxy, then you should not have to configure anything else on the WSA. (normally on browser, you will go for example http://www.example.com:81 to get to the server site that only accepts port 81 connection)
When the application runs, and if it is http, it should still use the proxy setting on the browser, but attach port 81 to the request.
The Ironport should receive this request, and then when trying to connect to the end server, will extract the port to connect to from the application configured port.
I am not sure how your setup is, but should work if above is true.
Regards,
Eric