public class Order : EntityBase
{

    public Guid ClientKey { get; set; }
    public Guid EmployeeKey { get; set; }
    public DateTime CreatedOn { get; set; }
    public DeliveryType DeliveryType { get; set; }
    public Address Address { get; set; }
    public Client Client { get; set; }
    public Employee Employee { get; set; }

    public bool SoftDeleted { get; set; }
    public virtual ICollection<JourneyOrder> JourneyOrder { get; set; }
    **public virtual ICollection<Item> Items
    { get; set; }**
    public virtual ICollection<ItemToBeFactored> ItemToBeFactored
    { get; set; }


    public Order() : base()
    {

        Items = new HashSet<Item>();
        ItemToBeFactored = new HashSet<ItemToBeFactored>();
        JourneyOrder = new HashSet<JourneyOrder>();
        CreatedOn = DateTime.Now;

    }
}

public class Item : EntityBase
{

    public string Name { get; set; }
    public double Height { get; set; }
    public double width { get; set; }
    public string Color { get; set; }
    public Guid OrderKey { get; set; }

    **public Order CustomerOrder { get; set; }**
}

var order = _context.Orders.Find(new Guid("67972aa1-2bb4-4916-b48b-1fdc3c2f5db9"));

        order.Items = new List<Item> {
            new Item {  Color = "Green", width = 20d }
        };


        **_context.SaveChanges();**