Question: what is the LINQ-to-Entity code to insert an order for a specific customer?
Notice that Order has a Customer property. You don't have to add the Order to the Customer -- you can do it the other way around. So, instead of creating a new Customer, get the Customer using Linq, then add it to your new Order.
using (OrderDatabase ctx = new OrderDatabase())
{
ctx.AddOrder(new Order()
{
OrderQuantity = 2,
OrderDescription = "Widgets",
Customer = ctx.Customers.First(c => c.CustomerId == yourId)
});
ctx.SaveChanges();
}