In your project config file add below keys:
<add key="ClientValidationEnabled" value="true" />
<add key="UnobtrusiveJavaScriptEnabled" value="true" />
Or if you want to set this programmatically then do below code in Application_Start()
void Application_Start(object sender, EventArgs e)
{
HtmlHelper.ClientValidationEnabled = true;
HtmlHelper.UnobtrusiveJavaScriptEnabled= true;
}