How Do I Pass A Javascript Variable Value Into Codebehing (C#)

You can do this by using AJAX call.
1.In the next example I am doing indirectly AJAX call by using jQuery, and I am sending 5 parameters with values from the current page controls:

function computeQuantity(computeUrl) {
    
    var itemComment1 = $("#itemComment").val();
    var itemAmount1 = $("#itemAmount").val();
    var itemLength1 = $("#itemLength").val();
    var itemWidth1 = $("#itemWidth").val();
    var itemThickness1 = $("#itemThickness").val();
    //
    $.post("/" + computeUrl + "/ComputeQuantity", { itemComment: itemComment1, itemAmount: itemAmount1, itemLength: itemLength1, itemWidth: itemWidth1, itemThickness: itemThickness1 },
        function (result) { $("#defineQuantity").html(result); });
    //
    return true;
}

2.In the next example I am using direct AJAX call for sending address parameter:

function CartAddressChanged() {
        var address = $("#_addressTypeDropDown").val();
        $.ajax({
            type: "POST",
            url: "/Order/CartForNewAddress",
            data: "address=" + address,
            success: function () {
                document.location = "/Order/ShoppingCart";
            },
            error: function (jqXHR, exception) {
                alert('Uncaught Error.\n' + jqXHR.responseText);
            }
        }); //end ajax call
        return false;
    };