1. Run Fiddler.
2. Run Fiddler and choose (from the menu) Tools, Fiddler Options…  Check these boxes in the HTTPS tab of the resulting dialog (and hit OK): 
3. Click Tools > Fiddler Options > Connections. Change the Fiddler Listen Port from 8888 to something else, e.g. 8889. (The Windows Phone team decided to steal port 8888 for their debugger).Set these options in the Connections tab and then hit OK: 
   
4. Set Proxy in the Gateway tab and then hit OK(this step is according the requirement. if required, then do this setting, else not):.
5. 
6. Click Ok button
7. In the QuickExec box below Fiddler‘s session list, type prefs set fiddler.network.proxy.RegistrationHostName 169.254.80.80 where the IP address is found at the top of the tooltip you see when hovering over the Online indicator at the top-right of Fiddler‘s toolbar.
8. Restart Fiddler.
9. Start VS 2013 and load your project. Click Run for the Windows Phone 8.1 emulator.
10. done.
11. 

reference:

http://blogs.msdn.com/b/wsdevsol/archive/2013/06/05/configure-the-windows-phone-8-emulator-to-work-with-fiddler.aspx

http://stackoverflow.com/questions/23462697/windows-phone-8-1-emulator-not-proxying-through-fiddler

Configure the Windows Phone 8.1 Emulator to work with Fiddler