var formData = jQuery('#yourID').serializeArray();
formadata.find(function(input){
	return input.name == 'yourFormName';
}).value = yourValueOrVariable;


jQuery.ajax({
	type: 'POST',
    url: 'your/url',
    dataType: 'json',
    data: formData,
    success: function(data){
	   //code here
    }
})