protected void RadGrid1_ItemCommand(object sender, GridCommandEventArgs e)
{
    var grid = (RadGrid)sender;

    if(e.CommandName == RadGrid.ExportToExcelCommandName)
    {
        var exportOutput = grid.MasterTableView.GenerateXlsxOutput<xlsx.Workbook>() as xlsx.Workbook;

        // Do something with output here.
    }
}

Protected Sub RadGrid1_ItemCommand(ByVal sender As Object, ByVal e As GridCommandEventArgs)
    Dim grid = CType(sender, RadGrid)

    If e.CommandName = RadGrid.ExportToExcelCommandName Then
        Dim exportOutput = TryCast(grid.MasterTableView.GenerateXlsxOutput(Of xlsx.Workbook)(), xlsx.Workbook)
        ' Do something with output here.
    End If
End Sub

<telerik:RadGrid ID="RadGrid1" runat="server" AllowPaging="True" OnItemCommand="RadGrid1_ItemCommand">
    <ExportSettings ExportOnlyData="true" IgnorePaging="true">
        <Excel Format="Xlsx" />
    </ExportSettings>
    <MasterTableView CommandItemDisplay="Top">
        <CommandItemSettings ShowExportToExcelButton="true" />
    </MasterTableView>
</telerik:RadGrid>

protected void RadButton1_Click(object sender, EventArgs e)
{
    GridItem myGridItem = null;

    if (RadGrid1.ShowHeader)
        myGridItem = RadGrid1.MasterTableView.GetItems(GridItemType.Header).FirstOrDefault();
    else if (RadGrid1.MasterTableView.CommandItemDisplay != GridCommandItemDisplay.None)
        myGridItem = RadGrid1.MasterTableView.GetItems(GridItemType.CommandItem).FirstOrDefault();
    else if (RadGrid1.ShowFooter)
        myGridItem = RadGrid1.MasterTableView.GetItems(GridItemType.Footer).FirstOrDefault();
    else if (RadGrid1.Items.Count > 0)
        myGridItem = RadGrid1.Items[0];

    if (myGridItem != null)
        myGridItem.FireCommandEvent("ExportToExcel", "");
}

Protected Sub RadButton1_Click(ByVal sender As Object, ByVal e As EventArgs)
    Dim myGridItem As GridItem = Nothing

    If RadGrid1.ShowHeader Then
        myGridItem = RadGrid1.MasterTableView.GetItems(GridItemType.Header).FirstOrDefault()
    ElseIf RadGrid1.MasterTableView.CommandItemDisplay <> GridCommandItemDisplay.None Then
        myGridItem = RadGrid1.MasterTableView.GetItems(GridItemType.CommandItem).FirstOrDefault()
    ElseIf RadGrid1.ShowFooter Then
        myGridItem = RadGrid1.MasterTableView.GetItems(GridItemType.Footer).FirstOrDefault()
    ElseIf RadGrid1.Items.Count > 0 Then
        myGridItem = RadGrid1.Items(0)
    End If

    If myGridItem IsNot Nothing Then myGridItem.FireCommandEvent("ExportToExcel", "")
End Sub