How to use the BeforeExport event of a GridView Extension

@using DevExpress.Razor.Models
@using System.Web.UI.WebControls
@{
    ViewBag.Title = "Home Page";
}

<script type="text/javascript">
    function ExportToPDF() {
        var names = GetSelectedItemsNames();
        if (!names) {
            alert("Choose columns to export");
            return;
        }
            
        document.getElementById("ExportColumnsNames").value = names;
        document.forms[0].submit();
    }
    function GetSelectedItemsNames() {
        var selectedItems = columnNames.GetSelectedValues();
        var result = "";
        for (var index = 0; index < selectedItems.length; index++) {
            result += selectedItems[index] + ";";
        }
        return result;
    }
</script>

@{
    string[] exportColumnsNames = ViewData["ExportColumnsNames"] == null 
        ? new string[] { "FirstName", "LastName", "HireDate" } 
        : ViewData["ExportColumnsNames"] as string[];
}

@using (Html.BeginForm("ExportTo", "Home")) {
    <div style="padding: 10px;">
        <br />
        @Html.DevExpress().Button(settings => {
            settings.Name = "exportToPdf";
            settings.Text = "Export to PDF";
            settings.ClientSideEvents.Click = "function(s, e) { ExportToPDF(); }";
        }).GetHtml()
        @Html.Hidden("ExportColumnsNames", null)
    </div>
    <div style="float:left;">
        @Html.DevExpress().Label(settings => {
           settings.Name = "title";
           settings.Text = "Export columns: ";
        }).GetHtml()
    </div>
    <div style="float:left; padding-left: 10px; padding-right: 10px;">
        @Html.DevExpress().ListBox(settings => {
            settings.Name = "columnNames";
            settings.Properties.SelectionMode = ListEditSelectionMode.CheckColumn;
            settings.Height = Unit.Pixel(225);
       
            foreach(string name in NorthwindDataProvider.GetColumnsNames()){
                settings.Properties.Items.Add(name);
                settings.Properties.Items.FindByText(name).Selected = exportColumnsNames.Contains(name);
            }
        }).GetHtml()
    </div>
    <div>
        @Html.Partial("BeforeExportPartial", NorthwindDataProvider.GetEmployees())
    </div>
}