WCF in .net core (TransportWithMessageCredential)

This has been fixed by the latest packages.

  <ItemGroup>
    <PackageReference Include="System.ServiceModel.Duplex" Version="4.6.0" />
    <PackageReference Include="System.ServiceModel.Http" Version="4.6.0" />
    <PackageReference Include="System.ServiceModel.NetTcp" Version="4.6.0" />
    <PackageReference Include="System.ServiceModel.Security" Version="4.6.0" />
  </ItemGroup>  

Actually found a valid workaround, there is a package you can use for this: https://github.com/gravity00/SimpleSOAPClient

using SimpleSOAPClient;
using SimpleSOAPClient.Handlers;
using SimpleSOAPClient.Helpers;
using SimpleSOAPClient.Models;
using SimpleSOAPClient.Models.Headers;

...

_client = SoapClient.Prepare().WithHandler(new DelegatingSoapHandler());
_client.HttpClient.DefaultRequestHeaders.Clear();
_client.HttpClient.DefaultRequestHeaders.Add("SOAPAction", "Action...");

 var requestEnvelope = SoapEnvelope
     .Prepare()
     .Body(request)
     .WithHeaders(KnownHeader.Oasis.Security.UsernameTokenAndPasswordText(Username, Password));

var responseEnvelope = _client.Send(Url, "CanNotBeEmpty", requestEnvelope);

Got it to work like this, as a charm...