I had a similar problem due to a password protected proxy server and couldn't find much in the way of information out there - hopefully this helps someone. I wanted to pick up the credentials as used by the customer's browser. However, the CredentialCache.DefaultCredentials and DefaultNetworkCredentials aren't working when the proxy has it's own username and password even though I had entered these details to ensure thatInternet explorer and Edge had access.
The solution for me in the end was to use a nuget package called "CredentialManagement.Standard" and the below code:
using WebClient webClient = new WebClient();
var request = WebRequest.Create("http://google.co.uk");
var proxy = request.Proxy.GetProxy(new Uri("http://google.co.uk"));
var cmgr = new CredentialManagement.Credential() { Target = proxy.Host };
if (cmgr.Load())
{
var credentials = new NetworkCredential(cmgr.Username, cmgr.Password);
webClient.Proxy.Credentials = credentials;
webClient.Credentials = credentials;
}
This grabs credentials from 'Credentials Manager' - which can be found via Windows - click Start then search for 'Credentials Manager'. Credentials for the proxy that were manually entered when prompted by the browser will be in the Windows Credentials section.