SubmissionTest.cs

[Fact]
public async Task ShouldSubmitCorrectPayload()
{
    // ... Do all the set up required i.e. stubs, domain objects
    // with pre-cooked data neccesary to execute the behaviour.
	
    // Build the service/adapter object with this mock HttpMessageHandler
    var loopbackHttpHandler = new SubmissionServiceHttpHandlerStub();
	
    var submitter = new SubmissionService(
        new HttpClient(loopbackHttpHandler)
        {
            BaseAddress = new Uri("http://foo.bar")
        });

    var useCase = CreateUseCase(submitter);
    useCase.Execute(...);

    // verify that what will have been submitted to the external 
    // service would have been the right data
    var payload = loopbackHttpHandler.PayloadDto("key");
    payload.Field1.Should().Be("abc");
    payload.Field2.Should().Be(10);
    // other assertions
}