Ok,
Let me help you on this,
1. Open "DHCP server" page ( More-->DHCP server)
2. Create a new local DHCP as shown bellow,
here type should be "Local", select Guest VLAN, enter guest subnet and mask
Here you can select the option type ( 252) and you can enter the path for the proxy .
3. map this dhcp to the Guest SSID, as shown bellow,
4. ensure that the proxy is configured in the IAP as shown bellow ,
Hope got more clarity on this,
Please feel free for any further help on this.