In this sample i want tosend variable Context.Request.Query through ajax before i post to controller.

@{
    Layout = null;
    ViewBag.Title = "Login";
    var returnUrl = @Context.Request.Query["returnurl"];
}

[HttpPost]
public async Task<IActionResult> Login(LoginModel model, string returnUrl = "")
{
    try
    {
        if (await _iusers.ValidateLogin(model))
        {
            return Json(new { User = "Valid"});
        }
        else
        {
            return Json(new { User = "NotValid" });
        }
    }
    catch (Exception ex)
    {
        throw ex;
    }
}

$(document).ready(function () {
    $("#btnlogin").on("click", function () {
        var jsonRequest = {
            Email: $("#logemail").val(),
            Password: $("#logpassword").val(),
        };
        var url = '@Url.Action("Login","Account")';
        $.ajax({
            url: url,
            type: "POST",
            data: { 'model': jsonRequest, 'returnUrl': 123 },
            dataType: "json",
            success: function (data) {
                if (data.user == "Valid") {
                    //alert("yes");
                    window.location.href = "@Url.Action("Index", "Home")";
                }
                else if (data.user == "NotValid") 
                {
                    alert("no");
                }
            },
            error: function (data) {
                alert("error");
            }
    });
});