>How can I put something in Clipboard clicking a button?
>(WebForm)

From the Help:

To place data on the Clipboard

Use the SetDataObject method to send data to the Clipboard.
The following example shows how to make the Click event of a Button control send the text of a TextBox control to the Clipboard:

' Visual Basic
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
   Clipboard.SetDataObject(TextBox1.Text)
End Sub

// C#
private void button1_Click(object sender, System.EventArgs e)
{
   Clipboard.SetDataObject(textBox1.Text);
}