CustomerControllerTests.cs

//Arrange
var mockCustomerService = new Mock();
var customerController = new CustomerController(mockCustomerService.Object);

Customer customerServiceSaveArg = null;
mockCustomerService
    .Setup(x => x.Save(It.IsAny()))
    .Returns(1)
    .Callback(c => customerServiceSaveArg = c);

//Act
var result = customerController.Post("Alex", "Brown");