How to take value from client side control from server side code?

Hi all,

I would like to know that, how to access client side control in server side code?

Here is an example..

I am having the textBox in client side like

<input type="text" id="textBoxUsername" name="textBoxUsername"/>

I need to take the value from this textBox in button click event which is server side code.

private void buttonShow(object sender, System.EventArgs e) {

//I need to access this textBox here.

}

How to do that?

Thanks

R. eswaran.
...