declare class ASPxClientGridViewCustomButtonEventArgs extends ASPxClientProcessingModeEventArgs

<dx:ASPxGridView ID="ASPxGridView1" runat="server" ClientInstanceName="gridView">
    <ClientSideEvents CustomButtonClick="grid_CustomButtonClick" />  
    <Columns>  
        <dx:GridViewCommandColumn VisibleIndex="0" Width="100px">  
            <CustomButtons>  
                <dx:GridViewCommandColumnCustomButton Text="Show New Window" ID="ShowNewWindow">  
                </dx:GridViewCommandColumnCustomButton>  
            </CustomButtons>  
        </dx:GridViewCommandColumn>  
        ...
    </Columns>  
</dx:ASPxGridView>

function grid_CustomButtonClick(s, e) {  
    if (e.buttonID != 'ShowNewWindow') return;  
        s.GetRowValues(e.visibleIndex, 'ID', ShowPopup);  
    }  
}  

function ShowWindow(Id) {  
    // your code
}