If the web server is only internal, and the proxy server just allow access to internet sites, then you cannot configure the web server to be accessed by the proxy server.
There are two ways you can try:
Configure the proxy server so that it can access your web server with internal IP.
Configure your browser so that it does not use proxy the your web server. For example with Internet Explorer:
- Call up the dialog "Internet Options"
- Goto "Connections" pane
- Click "LAN settings"
- Check "Bypass proxy server for local addresses"
- If this does not help, click "Advanced", and type the address/name of your web server in the edit box "Do not use proxy server for address beginning with"