how to get selected option value from javascript and pass it to server side on page redirect

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C#

ASP.NET

Javascript

i am writing a javascript code where when user clicks on other buttons or close the browser after data is filled by saved button is not clicked, a message is shown that tells user "if they wish to stay or exit. on exit all data will be lost". my javascript is .

<script type="text/javascript">
    var reconnect = false;
    window.onfocus = function () {
        if (reconnect) {
            reconnect = false;
            alert("Perform an auto-login here!");
        }
    };
    window.onbeforeunload = function () {
        var msg = "Are you sure you want to leave?";
        reconnect = true;
        return msg;

    };
</script>

this script provides option to either stay on page or leave page.. now when user clicks on leave page,how can i set a code here that records this option and send it in server side.. i am planning to implement hiddenfield in client side but what i am not clear is in how to set the option choosen by user in to hidden field.. any help or alternative..?? thanks in advance