Please try to store the information in an html hidden input.

<input id="ExportType" type="hidden" runat="server" />

On your client-side button click event, set the value of the html hidden input.

function Button1_onclick() {
    // retrieves WebGrid's object
    var WebGrid1 = ISGetObject("WebGrid1");
    var ExportTypeResult = document.getElementById("ExportType");
    ExportTypeResult.value = "EXCEL";
    WebGrid1.ExportGrid("", "EXCEL", "PORTRAIT"); // export grid to excell
}

And obtain the stored information in PrepareExportExecute server side event of WebGrid by using following code.

protected void WebGrid1_PrepareExportExecute(object sender, ISNet.WebUI.WebGrid.PrepareExportExecuteEventArgs e)
{
    string ExportResultType = ExportType.Value;
}

Hope this help.

Perfect, thank you!

Glad to hear the good news.

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