Plain Proxy Settings
By default, plain conventional HTTP proxy connections are accepted on port 8080 - a well-known port designated for HTTP proxies. The IP address to listen on is set as 0.0.0.0 meaning all IPv4 addresses assigned to the machine.
These settings can be adjusted by selecting the Proxy Settings menu in Admin UI / Web Filtering Proxy section and then by clicking the Standard Proxy node as indicated on the following screenshots.
Do not forget to Restart the Web Filtering Proxy service in order for the changes to be applied.