CustomerControllerTests.cs

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

mockCustomerService.Setup(x=> x.Save(It.IsAny())
    .Returns(1);

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

//Assert
mockCustomerService
    .Verify(x => x.Save(
        It.Is(
            c =>
                c.FirstName == "Alex" &&
                c.LastName == "Brown"
        )));