Persist data in related tables (1 x N)

I have these 2 related classes, and the requested class has a Collection of "OrderItems" ...

The Order class:

public class PedidoDTO
    {
        public int pedidoID { get; set; }
        public int codigo { get; set; }
        public DateTime data { get; set; }

        public int clienteID { get; set; }
        public virtual ClienteDTO Cliente { get; set; }

        public virtual IEnumerable<ItemPedidoDTO> itensPedidos { get; set; }
    }

And the OrderPrice class:

 public class ItemPedidoDTO
    {
        public int itemPedidoID { get; set; }
        public int quantidade { get; set; }
        public decimal preco { get; set; }
        public int porcentagemDesconto { get; set; }

        public int pedidoID { get; set; }
        public virtual PedidoDTO pedido { get; set; }

        public int produdoID { get; set; }
        public virtual ProdutoDTO produto { get; set; }
    }

My question is: How do I persist the data in a way that the RequestMessage automatically receives the foreign key?

In the project I make use of: Code first, EF6, migrations, generic repositories, implementation repositories and mapping with Fluent Api.