You can place a piece of script, html or iframe in your page by means of the 'Widget' option.
In the page you can choose 'insert' > 'content' (4th icon on the right of the screen). Then choose the option 'widget' in the selection list.
You can give the widget a name and enter the code here.
Placing an iframe as a widget
If you only have a URL that you want to show in the widget - you can do this by 'building' a bit of script around the URL. You can use the following code for that;
<iframe style="height: 100%; width: 100%" frameborder="no" src="https://www.ziber.nl"></iframe>
You can replace the entire URL in this example (https://www.ziber.nl) with the URL you want in the iframe.