Passing information from view to controller

I am working on an asp-mvc application and am facing the following problem: I have a model with simple properties plus one property that is a list of my custom object and I am providing an Ienumerable property as stated here: Passing a model's IEnumerable property to a controller post action - ASP MVC

In my opinion, I have a button that is supposed to add items to the ienumerable property of my model. Of course, I don't want to lose the data already inserted, so I need to pass the model to the appropriate action.

I noticed that the os model is completely portable only to mail. So I did something like:

 $(".addButton").click(function (event) {
        event.preventDefault();
        $("#FilterForm").submit();
        @{ Session["fromAddFullItem"] = "true";}
        return false;
      });

      

        
        
        
      

    

And then in my controller I do something like:

public ActionResult Index(FilterModel model)
    {
        if (Session["fromAddFullItem"].ToString() == "true")
        {
            Session["fromAddFullItem"] = "false";
            return AddBlankItemTemplate(model);
        }

      

        
        
        
      

    

I read that assigning a session in js is not recommended but also tried TempData and the data was always empty there.

My problem is that Session ["fromAddFullItem"] is always right, even when I come from another button. If I put a breakpoint in addbtn, click in the -Session line ["fromAddFullItem"] = "false"; and click another button, I see that for some odd reason the mentioned breakpoint hit, even though I didn't click the add button.

Any help? Perhaps there is another way to achieve what I want. Currently, no matter which button I click (which submits the form) it comes in as Session ["fromAddFullItem"] = "false" and goes to the AddBlankItemTemplate action. Thank.

EDIT - AJAX POST

    $(".addButton").click(function(event) {
        event.preventDefault();
        var modelData = JSON.stringify(window.Model);
        $.ajax({
            url:  '@Url.Action("AddBlankItemTemplate")',
            type: 'POST',
            dataType: 'json',
            data: modelData,
            contentType: 'application/json; charset=utf-8',

        });
        return false;
    });

      

        
        
        
      

    

and controller

public ActionResult AddBlankItemTemplate(string modelData)

      

        
        
        
      

    

EDIT 2:

       $(".addButton").click(function (event) {
        event.preventDefault();
        $.ajax({
            url: '@Url.Action("AddBlankItemTemplate")',
            data: $("#FilterForm").serialize()
        }).success(function(partialView) {

            $('DetailsTemplates').append(partialView);
        });
    });

      

        
        
        
      

    

and the controller:

public ActionResult AddBlankItemTemplate(FilterModel model)