C# :: List Initialize In Object Initialization Method

public class Customer
{
public int Id { get; set; }
public string Name { get; set; }
public List<Order> Orders { get; set; }
public Customer() {
Orders = new List<Order>();
}
}

This object is called as

var cust = new Customer
{
Id = 1, Name = "Khatana", [b]Orders.Add(new Order())[/b]
}

I want to ask, Orders.Add(new Order()) is a wrong way, but why..!!... as List<Order> already has been initialized in Customer constructor, then why it is again required to be initialized in object initialization.
Is this a correct way

List<Order> orders = new List<Order> {aaa.....bb....cc};
var cust = new Customer
{
id = 1, Name = "Khatana", Orders = new List<Order> { orders }
}

Why we need to initialize a list in an object initialization, where as the list has been already initialized in object constructor.