var sample1 = "aaa";
var sample2 = "bbb";

    $.ajax({

   url: "Action/Controller",
   data: $("#form123").serialize(),
   cache: true,
   type: "POST",
   dataType: 'html',
   success: function (data) {
   $('#form123').html(data)};

});

data: {
  formData: $("#form123").serialize(),
  sample1: sample1,
  sampleTwo: sample2
}

var formData = $("#form123").serialize() + '&' + sample1 + '&' + sample2;
$.ajax({
  ...
  data: formData,
  ...
});

$.ajax({
   url: "Action/Controller/" + sample1 + "/" + sample2,
   data: $("#form123").serialize(),
   cache: true,
   type: "POST",
   dataType: 'html',
   success: function (data) {
   $('#form123').html(data)};

});

@using(Ajax.BeginForm("Action", "Controller", new {@param1 = "aaa", @param2 = "bbb"}, new AjaxOptions(){HttpMethod = "Post", UpdateTargetId = "#section1"})){
<div id="#section1">

..... put your form controls here . . .

</div>
}

public PartialViewResult Action(string param1, string param2, ViewModel model)

<input type="hidden" id="var1" name="var1" value="@sample1" />

<input type="hidden" id="var2" name="var2" value="" />

$('#var2').val(var2); //before sending your form of course!