Unable to get the value in Controller after using jquery in asp.net core

i am new to Ap.net core MVC I am in middle of a project.. i am trying to create a url using jquery and pass that value to controller on form submit.. i can see the value being displayed after i execute a function on button click. I am using loading a partial view on dropdown using jquery im able to populate it and also assign the value to the textbox. But when i try to access the same in controller im not able to find it below is my complete code. I am able to access all the other values but the textbox value which setting through jquery

Below is my View

<form method="post" asp-action="Upsert" enctype="multipart/form-data">

<div class="row px-2 mx-2 backgroundWhite border">
    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
    @if (Model.NavId != 0)
    {
        <input type="hidden" asp-for="NavId" />
        title = "Edit Navigation";
    }

    <div class="col-12">
        <h2 class="text-primary">@title</h2>
        <hr class="my-4">
    </div>
    <div class="col-8">
        <div class="form-group row">
            <div class="col-5">
                <label asp-for="Title"></label>
            </div>
            <div class="col-7">
                <input asp-for="Title" class="form-control" />
                <span asp-validation-for="Title" class="text-danger"></span>
            </div>
        </div>
        <div class="form-group row">
            <div class="col-5">
                <label asp-for="Position"></label>
            </div>
            <div class="col-7">
                <input asp-for="Position" class="form-control" />
                <span asp-validation-for="Position" class="text-danger"></span>
            </div>
        </div>
        <div class="form-group row">
            <div class="col-5">
                <label asp-for="NavigateUrl">NavigationUrl</label>
            </div>
            <div class="col-7">
                <input asp-for="NavigateUrl" disabled id="NavigateUrl"  name="NavigateUrl" class="form-control"   style="color:#000 !important"/>
                <span asp-validation-for="NavigateUrl" class="text-danger"></span>

            </div>
        </div>
        <div class="form-group row">
            <div class="col-5">
                <label>Select Page</label>
            </div>
            <div class="col-7">
                <select id="SelectPage" class="form-control">
                    <option value="-">Select Page</option>
                    <option value="Products">Products</option>
                    <option value="ProductDetails">Product Details</option>
                </select>

            </div>
        </div>

    </div>

    <div class="col-12 selectDiv mt-2 mb-2">

    </div>
    <div class="form-group row col-8">
        <div class="col-12 offset-4">
            @if (Model.NavId != 0)
            {
                //Edit button and back to list
                <partial name="_EditAndBackToListButton" model="Model.NavId" />
            }
            else
            {
                //Create button and back to list
                <partial name="_CreateAndBackToListButton" />
            }
        </div>
    </div>

</div>

@section Scripts{

    <script>
        $(document).ready(function () {
            $('#SelectPage').change(function () {
                var selectedvalue = $(this).children("option:selected").val();
                    $(".selectDiv").load("/Admin/Nav/GetCategoryOrProducts",
                        { option: selectedvalue });
            });
        });

    </script>
    <script>
        var assignvalue = function (primaryId) {

            event.preventDefault();

            var selectedvalue = $('#SelectPage').children("option:selected").val();
            var navtext = $('#NavigateUrl');
            var navurl = "/" + selectedvalue + "/" + primaryId;
            navtext.val(navurl);

        };
    </script>
    @{ await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

My Controller method

    [HttpPost]
    [ValidateAntiForgeryToken]
    public IActionResult Upsert(Nav nav)
    {
         if (ModelState.IsValid)
        {
            if (nav.NavId == 0)
            {
                string navurl = HttpContext.Request.Form["NavigateUrl"].ToString(); // I am not able to get the value here it shows null
                nav.NavigateUrl = navurl;
                _unitOfWork.Nav.Add(nav);
            }
            else
            {
                _unitOfWork.Nav.Update(nav);
            }
            return RedirectToAction(nameof(Index));

        }
        return View();
    }