$('#myformelement').append('<input type="hidden" name="myfieldname" value="myvalue" />');

$('<input>').attr('type','hidden').appendTo('form');

回答你的第二个问题:

$('<input>').attr({
    type: 'hidden',
    id: 'foo',
    name: 'bar'
}).appendTo('form');

$('#myformelement').append('<input type="hidden" name="myfieldname" value="myvalue" />');

function addHidden(theForm, key, value) {
    // Create a hidden input element, and append it to the form:
    var input = document.createElement('input');
    input.type = 'hidden';
    input.name = key; //name-as-seen-at-the-server
    input.value = value;
    theForm.appendChild(input);
}

// Form reference:
var theForm = document.forms['detParameterForm'];

// Add data:
addHidden(theForm, 'key-one', 'value');

和David的一样，但是没有attr()

$('<input>', {
    type: 'hidden',
    id: 'foo',
    name: 'foo',
    value: 'bar'
}).appendTo('form');

工作JSFIDDLE

如果你的形式是

<form action="" method="get" id="hidden-element-test">
      First name: <input type="text" name="fname"><br>
      Last name: <input type="text" name="lname"><br>
      <input type="submit" value="Submit">
</form> 
    <br><br>   
    <button id="add-input">Add hidden input</button>
    <button id="add-textarea">Add hidden textarea</button>

你可以像这样添加隐藏输入和文本区域

$(document).ready(function(){

    $("#add-input").on('click', function(){
        $('#hidden-element-test').prepend('<input type="hidden" name="ipaddress" value="192.168.1.201" />');
        alert('Hideen Input Added.');
    });

    $("#add-textarea").on('click', function(){
        $('#hidden-element-test').prepend('<textarea name="instructions" style="display:none;">this is a test textarea</textarea>');
        alert('Hideen Textarea Added.');
    });

});

在这里检查工作jsfiddle