Call .ashx handler from button click event

ashx handler to download some files when user clicks a button. My .aspx looks like this right now

<asp:Button OnClick:"Download" runat="server" />

And within the Download event I have to do some processing before the download starts. So I'm looking for a way to invoke the handler from my C# code and pass in some parameters that are required for the download process.

Thanks in advance.