HttpClient with Proxy

I am unable to make web requests in unity targeting mono with a configured proxy. I am using HttpClient and Unity 2019.2.18f1. When running this exact same code in a .NET core console app, I have no problems. When I run this code in unity, I am greeted with this error:

WebException: (407) Proxy Authentication Required

I have verified the DefaultCredentials objects are the same in my console app and in unity editor. In fact, the proxy is even wide open.

Here is the sample code:

         var clientHandler = new HttpClientHandler();
         clientHandler.UseProxy = true;
         clientHandler.Proxy = new WebProxy("http://myproxy:3128");
         clientHandler.Proxy.Credentials = CredentialCache.DefaultCredentials;
         clientHandler.UseDefaultCredentials = true;

         var httpclient = new HttpClient(clientHandler);
         var result = httpclient.GetStringAsync("https://google.com").Result;
  

I have tried tried targeting .NET Framework 4.X and .Net standard 2.0 with no luck. I have set HTTP_proxy and HTTPS_proxy environment variables with no luck. I have tried using UnityWebRequest, HttpWebRequest, and WebClient with no luck. I have tried editor and standalone builds with no luck. I have tried manually setting headers with no luck.

How is this supposed to work in an enterprise environment where users have to specify proxy creds? Has anybody solved this? Am i doing something wrong?