Web servers (as others) can listen for connections on just about any port.  
If you need to monitor a web server on a TCP port other than 80, follow the steps described in HT0011:  Monitor a basic web server

The exception will be the final step of changing the Port value from WEB PAGE to the port number on which your web server is accepting connections (i.e. 8080 ) and changing the IP Address field appropriately to a real IP Address or FQDN.

Note: If you enter the port number  in the port field before hitting the Auto Configure button, you may get a message regarding the fact that no auto config parameters exist for that port.  The reason for this is simple, the port number used is non-standard and could therefore be ANY server or process.