HiddenFor - Submit loses value the first time

Follow the code below:

Index:

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

<div class="modal fade" id="minhaModal" tabindex="-1" role="dialog" aria-hidden="true">
    <div class="modal-dialog modal-lg" role="document">
        <div class="modal-content">
            <div id="conteudoModal"></div>
        </div>
    </div>
</div>

<div class="row">
    <button class="btn btn-default" id="myclick">click me</button>
</div>


<script src="~/Scripts/jquery-1.10.2.js"></script>
<script src="~/Scripts/jquery.unobtrusive-ajax.js"></script>

<script type="text/javascript">

    $("#myclick").click(function () {
        $("#conteudoModal").load("@Url.Action("_minhaPartialView", "Home")", function () {
            $("#minhaModal").modal("show");
        });
    });
</script>

_minhaPartialView:

@model WebApplication1.Models.Modelos

@using (Ajax.BeginForm("MyAction", "Home", new AjaxOptions { HttpMethod = "POST", OnBegin = "OnBegin_Function", OnSuccess = "OnSuccess_Function" }, new { @class = "form-horizontal", role = "form" }))
{
    <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
        <h4 class="modal-title">Título</h4>
    </div>

    @Html.AntiForgeryToken()
    @Html.HiddenFor(model => model.MyProperty)

    <div class="modal-footer">
        <button type="submit" class="btn btn-success">Salvar</button>
        <button type="button" class="btn btn-danger" data-dismiss="modal">Cancelar</button>
    </div>
}

<script type="text/javascript">

 function OnBegin_Function() {
     $('#MyProperty').val(123);
    }

    function OnSuccess_Function() {

    }

</script>

Controller:

[HttpGet]
public ActionResult _minhaPartialView()
{
    var model = new Modelos
    {
        MyProperty = 1
    };

    return PartialView(model);
}

public ActionResult MyAction(Modelos model)
{
    // aqui retorna model.MyProperty = 1 na primeira vez.
    // o certo é retornar valor 123, por exemplo
    return Json(true, JsonRequestBehavior.AllowGet);
}

Problem:

When I submit, the value returns 1 the first time. The second time returns value 123.

UPDATE:

$(function () {
    $('#MyProperty').val(123);  //AQUI FUNCIONA !!!
});

function OnBegin_Function() {  //NÃO FUNCIONA
    $("#MyProperty").val(123);      
}