I\'ve this simple form:
you can do this without jQuery:
var form=document.getElementById('form-id');//retrieve the form as a DOM element
var input = document.createElement('input');//prepare a new input DOM element
input.setAttribute('name', inputName);//set the param name
input.setAttribute('value', inputValue);//set the value
input.setAttribute('type', inputType)//set the type, like "hidden" or other
form.appendChild(input);//append the input to the form
form.submit();//send with added input