public async void TestGetAccessToken()
        {
            const string TOKEN              = "ABC";
            const string URI                = "https://example.com/token";
            var          facmoq             = StringClientFactoryMock(TOKEN, out var handlerMock);
            var          gegevensdienstMock = new Mock <IGegevensdienst>();

            gegevensdienstMock.Setup(geg => geg.TokenEndpointUri).Returns(new System.Uri(URI));

            var t = await PGOOAuth.GetAccessToken(
                gegevensdienst : gegevensdienstMock.Object,
                authorizationCode : "test123",
                redirectUri : "https://pgo.example.com/redir",
                httpClientFactory : facmoq
                );

            Assert.Equal(TOKEN, t);

            handlerMock.Protected().Verify(
                "SendAsync",
                Times.Exactly(1),
                ItExpr.IsAny <HttpRequestMessage>(),
                ItExpr.IsAny <CancellationToken>()
                );

            handlerMock.Protected().Verify(
                "SendAsync",
                Times.Exactly(1),
                ItExpr.Is <HttpRequestMessage>(req => req.Method == HttpMethod.Post && req.RequestUri.ToString().StartsWith(URI)),
                ItExpr.IsAny <CancellationToken>()
                );
        }