IEditableCollectionView.AddNew Method

            IEditableCollectionView editableCollectionView = 
                itemsControl.Items as IEditableCollectionView; 

            if (!editableCollectionView.CanAddNew)
            {
                MessageBox.Show("You cannot add items to the list.");
                return;
            }

            // Create a window that prompts the user to enter a new
            // item to sell.
            ChangeItemWindow win = new ChangeItemWindow();

            //Create a new item to be added to the collection.
            win.DataContext = editableCollectionView.AddNew();

            // If the user submits the new item, commit the new
            // object to the collection.  If the user cancels 
            // adding the new item, discard the new item.
            if ((bool)win.ShowDialog())
            {
                editableCollectionView.CommitNew();
            }
            else
            {
                editableCollectionView.CancelNew();
            }