This configuration is allow for firmware 17.1 and after.
How you can switch to using the device hostname for the Captive Portal and other end-user interactions as follows:
Log in to the Firewall console over ssh
Select option 4 (Device Console)
Enter the following command at the prompt: (use the command)
set http_proxy proxy_url_use_hostname on
To confirm that it has set, enter the following command:
Type ‘exit’ to return to the menu and logout.