<system.webServer>
        <httpProtocol>
            <customHeaders>
            <add name="X-Frame-Options" value="SAMEORIGIN" />
         </customHeaders>
     </httpProtocol>
</system.webServer>

private void Page_Load(object sender, System.EventArgs e)
{
   Response.AddHeader("X-Frame-Options", "SAMEORIGIN");
}