If a browser is configured to detect proxy server settings automatically, the proxy server address is never displayed anywhere in the browser. You may have an application, however, that requires you to explicitly define the proxy server to use. To determine the server to use, you can use PowerShell.
The URL to use in the query can be anything as long as it is one that will evaluate to using a proxy server, so don’t use an internal address that will evaluate to connecting directly. The query will return the DNS name of the proxy server and the port to use, e.g., proxy.company.com:8080. If you want these values separately, you can use the Host and Port properties.