Step 1: Create view model 
public class OrderVM
{
    public string OrderNo { get; set; }
    public DateTime OrderDate { get; set; }
    public string Description { get; set; }
    public List<OrderDetail> OrderDetails {get;set;}
}
Step 2: Add javascript for add order lines 
<script src="//code.jquery.com/ui/1.11.4/jquery-ui.js"></script>
    $(function () {
        $('#orderDate').datepicker({
            dateFormat : 'mm-dd-yy'
        });
    });
    $(document).ready(function () {
        var orderItems = [];
        //Add button click function
        $('#add').click(function () {
            //Check validation of order item
            var isValidItem = true;
            if ($('#itemName').val().trim() == '') {
                isValidItem = false;
                $('#itemName').siblings('span.error').css('visibility', 'visible');
            }
            else {
                $('#itemName').siblings('span.error').css('visibility', 'hidden');
            }
            if (!($('#quantity').val().trim() != '' && !isNaN($('#quantity').val().trim()))) {
                isValidItem = false;
                $('#quantity').siblings('span.error').css('visibility', 'visible');
            }
            else {
                $('#quantity').siblings('span.error').css('visibility', 'hidden');
            }
            if (!($('#rate').val().trim() != '' && !isNaN($('#rate').val().trim()))) {
                isValidItem = false;
                $('#rate').siblings('span.error').css('visibility', 'visible');
            }
            else {
                $('#rate').siblings('span.error').css('visibility', 'hidden');
            }
            //Add item to list if valid
            if (isValidItem) {
                orderItems.push({
                    ItemName: $('#itemName').val().trim(),
                    Quantity: parseInt($('#quantity').val().trim()),
                    Rate: parseFloat($('#rate').val().trim()),
                    TotalAmount: parseInt($('#quantity').val().trim()) * parseFloat($('#rate').val().trim())
                });
                //Clear fields
                $('#itemName').val('').focus();
                $('#quantity,#rate').val('');
            }
            //populate order items
            GeneratedItemsTable();
        });
        //Save button click function
        $('#submit').click(function () {
            //validation of order
            var isAllValid = true;
            if (orderItems.length == 0) {
                $('#orderItems').html('<span style="color:red;">Please add order items</span>');
                isAllValid = false;
            }
            if ($('#orderNo').val().trim() == '') {
                $('#orderNo').siblings('span.error').css('visibility', 'visible');
                isAllValid = false;
            }
            else {
                $('#orderNo').siblings('span.error').css('visibility', 'hidden');
            }
            if ($('#orderDate').val().trim() == '') {
                $('#orderDate').siblings('span.error').css('visibility', 'visible');
                isAllValid = false;
            }
            else {
                $('#orderDate').siblings('span.error').css('visibility', 'hidden');
            }
            //Save if valid
            if (isAllValid) {
                var data = {
                    OrderNo: $('#orderNo').val().trim(),
                    OrderDate: $('#orderDate').val().trim(),
                    //Sorry forgot to add Description Field
                    Description : $('#description').val().trim(),
                    OrderDetails : orderItems
                }
                $(this).val('Please wait...');
                $.ajax({
                    url: '/Home/SaveOrder',
                    type: "POST",
                    data: JSON.stringify(data),
                    dataType: "JSON",
                    contentType: "application/json",
                    success: function (d) {
                        //check is successfully save to database 
                        if (d.status == true) {
                            //will send status from server side
                            alert('Successfully done.');
                            //clear form
                            orderItems = [];
                            $('#orderNo').val('');
                            $('#orderDate').val('');
                            $('#orderItems').empty();
                        }
                        else {
                            alert('Failed');
                        }
                        $('#submit').val('Save');
                    },
                    error: function () {
                        alert('Error. Please try again.');
                        $('#submit').val('Save');
                    }
                });
            }
        });
        //function for show added items in table
        function GeneratedItemsTable() {
            if (orderItems.length > 0) {
                var $table = $('<table/>');
                $table.append('<thead><tr><th>Item</th><th>Quantity</th><th>Rate</th><th>Total</th></tr></thead>');
                var $tbody = $('<tbody/>');
                $.each(orderItems, function (i, val) {
                    var $row = $('<tr/>');
                    $row.append($('<td/>').html(val.ItemName));
                    $row.append($('<td/>').html(val.Quantity));
                    $row.append($('<td/>').html(val.Rate));
                    $row.append($('<td/>').html(val.TotalAmount));
                    $tbody.append($row);
                });
                $table.append($tbody);
                $('#orderItems').html($table);
            }
        }
    });
</script>
Step 3: Create an action for save data 
[HttpPost]
    public JsonResult SaveOrder(OrderVM O)
    {
        bool status = false;
        if (ModelState.IsValid)
        {
            using (MyDatabaseEntities dc = new MyDatabaseEntities())
            {
                Order order = new Order { OrderNo = O.OrderNo, OrderDate = O.OrderDate, Description = O.Description };
                foreach (var i in O.OrderDetails)
                {
                    //
                   // i.TotalAmount = 
                    order.OrderDetails.Add(i);
                }
                dc.Orders.Add(order);
                dc.SaveChanges();
                status = true;
            }
        }
        else
        {
            status = false;
        }
        return new JsonResult { Data = new { status = status} };
    }
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