JQuery add hidden element to form programmatically

try

$('#someid').append($('<input></input>').attr('id','hidden1').attr('type','hidden').attr('value','some val'));

You can create a jQuery function to do most of the work for you. I use this function to add hidden inputs programatically:

jQuery Function:

// This must be applied to a form (or an object inside a form).
jQuery.fn.addHidden = function (name, value) {
    return this.each(function () {
        var input = {};
        if (Object.prototype.toString.call(value) === '[object Array]') {
            var r = /\[\]/;
            // Pass an array as a series of separate hidden fields.
            for (var i = 0; i < value.length; i++) {
                input = $("<input>").attr("type", "hidden")
                                    .attr("name", name.replace(r, '[' + i + ']'))
                                    .val(value[i]);
                $(this).append($(input));
            }
        } else {
            input = $("<input>").attr("type", "hidden")
                                .attr("name", name)
                                .val(value);
            $(this).append($(input));
        }
    });
};

Usage:

For standard form items or simple parameters in MVC theHidden as String:

$('#myform').addHidden('theHidden', 'jam');
=> <input type="hidden" name="theHidden" value="jam">

For list parameters in MVC ID As List(Of Integer):

$('#myform').addHidden('ID', [1,2,5]);
=> <input type="hidden" name="ID" value="1">
   <input type="hidden" name="ID" value="2">
   <input type="hidden" name="ID" value="4">

For complex types in MVC which have a List property model As ComplexType:

$('#myform').addHidden('Customer[].CustomerID', [1,2,5]);
=> <input type="hidden" name="Customer[0].CustomerID" value="1">
   <input type="hidden" name="Customer[1].CustomerID" value="2">
   <input type="hidden" name="Customer[2].CustomerID" value="5">

Class ComplexType
    Property Customer As List(Of CustomerDetail)
End Class
Class CustomerDetail
    Property CustomerID As Integer
End Class

You need to insert the input ELEMENT into the DOM, not just inject HTML into the page. When it comes for form fields, there is a difference.

Use var field = $(document.createElement('input')), then set the attributes of the field.