407 authentication errors behind corporate firewall

[koolraap]

I can't use Ref12 behind an authenticating proxy:

18/11/2015 8:58:28 AM: Ref12: Errors occurred while trying all reference URLs; Ref12 will not work
System.Net.Http.HttpRequestException: Response status code does not indicate success: 407 (Proxy Authentication Required).
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at SLaks.Ref12.Services.ReferenceSourceProvider.<LookupService>d__9.MoveNext()