Actually it is NOT possible to use CP´s Network Payload to deploy a network setting with proxy auto discovery. You can select "Automatic" and have to provide the Proxy Server URL/PAC URL (requiered) witch sets the auto proxy url enabled.
From the Apple Profile Reference the pac url should be an option. So leaving the url field empty will set the network service proxy to proxy auto discovery using WPAD. Using the pac url will set the network service proxy to auto proxy url.
Apple Configuration Profile Ref If the ProxyType field is set to Auto and no ProxyPACURL value is specified, the device uses the web proxy autodiscovery protocol (WPAD) to discover proxies
Make the field Proxy Server URL optional!