Using Moq to mock an asynchronous method for a unit test

Recommend @Stuart Grassie's answer above.

var moqCredentialMananger = new Mock<ICredentialManager>();
moqCredentialMananger
                    .Setup(x => x.GetCredentialsAsync(It.IsAny<string>()))
                    .ReturnsAsync(new Credentials() { .. .. .. });

You're creating a task but never starting it, so it's never completing. However, don't just start the task - instead, change to using Task.FromResult<TResult> which will give you a task which has already completed:

...
.Returns(Task.FromResult(new HttpResponseMessage(System.Net.HttpStatusCode.OK)));

Note that you won't be testing the actual asynchrony this way - if you want to do that, you need to do a bit more work to create a Task<T> that you can control in a more fine-grained manner... but that's something for another day.

You might also want to consider using a fake for IHttpClient rather than mocking everything - it really depends on how often you need it.

With Mock.Of<...>(...) for async method you can use Task.FromResult(...):

var client = Mock.Of<IHttpClient>(c => 
    c.PostAsync(It.IsAny<Uri>(), It.IsAny<HttpContent>()) == Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK))
);