There are a few things to check before adding a website.
For technical and security reasons we can only support website with https:// or websites with SSL. So any websites with no SSL or has only http:// prefix will not return anything.
Secondly, our player loads external websites into an iframe (a sandboxed one for security reasons). The page you're trying to add may have an 'X-Frame-Options' header set to 'SAMEORIGIN' which means we can't load it into our iFrame. Here's how to check if your website has an X-Frame-Options in the header. In order to allow your website to load inside our player, you need to remove the 'X-Frame-Options' or allow *.screen.cloud in the option.
More on X-Frame-Options
Internal websites should work as long as the device is within the same network where the internal site resides.