ASP.NET Web API Windows authentication get username on an impersonated call

Question

For some reason I can't get the Windows username on an impersonated call.

I have followed the answers here but not getting the same result.

On server A I have a intranet ASP.NET MVC application with only Windows authentication in IIS.

In the web.config for the ASP.NET MVC application on server A, I use impersonation.

Code for calling Web API:

var impersonationContext = WindowsIdentity.GetCurrent().Impersonate();

using (impersonationContext)
{
    var client = GetHttpClient();
    return await client.PostAsync("services/ExecuteCommand/Execute", httpContent);
}

private HttpClient GetHttpClient()
{
   var httpClientHandler = new HttpClientHandler
   {
      UseDefaultCredentials = _commandServiceUseDefaultCredentials
   };
   var client = new HttpClient(httpClientHandler)
   {
      BaseAddress = new Uri(ConfigurationManager.AppSettings["CommandServiceBaseUrl"].ToString()),
   };
   client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));       
   return client;
}

On server B, I have a ASP.NET Core Web API project with Windows authentication

In the web.config for the Web API on server B a specify using Windows authentication, but NOT impersonation.

I'm trying to get the windows username for the person browsing my ASP.NET Core MVC application. I have tried a lot so I ended up with this method to get them all.

private string GetUserName()
{
   var windowsIdentity = System.Security.Principal.WindowsIdentity.GetCurrent();
   System.Security.Principal.WindowsIdentity windowsIdentity2 = null;
   if (System.ServiceModel.OperationContext.Current != null)
      if (System.ServiceModel.OperationContext.Current.ServiceSecurityContext != null)
         windowsIdentity2 = System.ServiceModel.OperationContext.Current.ServiceSecurityContext.WindowsIdentity;

   var httpContextIdentity = System.Web.HttpContext.Current.User.Identity;
   return string.Format("{0}_{1}_{2}_{3}_{4}",
                System.Environment.UserName ?? "",
                User.Identity.Name,
                windowsIdentity != null ? windowsIdentity.Name : string.Empty,
                windowsIdentity2 != null ? windowsIdentity2.Name : string.Empty,
                httpContextIdentity != null ? httpContextIdentity.Name : string.Empty);
}

This yields this result

System.Environment.UserName: ServerB App_pool user without domain

User.Identity.Name: Domain\ServerA$

System.Security.Principal.WindowsIdentity.GetCurrent().Name: ServerB domain\app_pool user

System.ServiceModel.OperationContext.Current.ServiceSecurityContext.WindowsIdentity.Name: (null)

System.Web.HttpContext.Current.User.Identity: Domain\ServerA$

So how can I get the windows username for the end user of my ASP.NET Core MVC app?