The server-side event handler has an extra parameter named as “parameter”.
This is the parameter send by the Java Script function.
Moreover you can send parameters to the code-behind too.
Download the control (link at the end of the article) and add it to your toolbox.
Visual Studio will automatically add Ajax Control Toolkit reference to your web site as well.
The Update Panel Java Script Extender is build on Ajax Control Toolkit Extender base libraries.