MVC5 Retreive data from view

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C#

MVC

I'm just starting out with MVC, done a quite a bit of WebForms so not a noob.
So far MVC is fairly intuitive and I'm catching on pretty quick but I'm stuck on one critical area;

I've set up a bootstrap-tagsinput[^] control on my view page and I'm able to collect the data in a JQuery function.

I've set up a ViewModel class and can pass values into and show them on the control but I'm stuck as to how to return these values to the controller??

Here's my view code;

@model JaxCoderV20.Models.PostViewModel

@{
    ViewBag.Title = "Home Page";
}

@using (Ajax.BeginForm("Calculate", "Home",
                            new AjaxOptions { UpdateTargetId = "divInterestDeatils" }))
{
    <div class="page-body">
        <div class="row">
            <div class="col-lg-6 col-sm-6 col-xs-12">
                <div class="well with-header">
                    <div class="header bordered-darkorange">Bootstrap Tags Input</div>
                    <div>
                        <input type="text" value="@Html.Encode(Model.TestStringData) " data-role="tagsinput" placeholder="Add tags" />
                    </div>
                    <div class="form-group">
                        <div class="col-md-offset-2 col-md-10">
                            <input type="submit" value="Create" class="btn btn-default" />
                        </div>
                    </div>
                </div>
            </div>
        </div>
    </div>
}
@section PageScripts{
    <!--Bootstrap Tags Input-->
    <script src="/assets/js/tagsinput/bootstrap-tagsinput.js"></script>
    <script>
        $( document ).ready(function() {
            $('form').on('submit',function()
            {   
                @Model.TestStringData = $('input').tagsinput('items')
            });
        });
    </script>
}

Then in the controller I have;

[HttpPost]
public ActionResult Calculate(PostViewModel model)
    {
       return Content("This is a test");
     }
}

and just for good measure here's my PostViewModel code;

namespace JaxCoderV20.Models
{
    public class PostViewModel
    {
        public Post PostData { get; set; }
        public Message MsgData { get; set; }
        public List<string> TestData { get; set; }
        public string TestStringData { get; set; }
    }
}

I have a break on the post and it happens but all fields in the PostViewControl are null.
I also put a break at the script and there is valid data there.

Thanks