Syntax: the full URL to a web proxy server
This specifies the URL (not just hostname) of a proxy web server through which to pass page fetch requests. Only host and (optional) port are used from the given URL. If empty, no proxy is used: pages are fetched directly.
Note that Proxy Auto-Config URL overrides this.
In Windows Control Panel, the Internet Options → Connections tab → LAN settings → Use a proxy server for your LAN host/port value is equivalent to the Search Appliance's Proxy setting.