<!DOCTYPE html>
<html>
<head>
<script  runat="server">
Sub Button1_Click(sender As Object, e As EventArgs)
      Response.Write("The ClientID of the button is: ")
      Response.Write(button1.ClientID)
End Sub
</script>

</head>
<body>

<form runat="server" >
<asp:Button ID="button1" OnClick="Button1_Click" Text="Get ClientID" runat="server" />
</form>

</body>
</html>