I ran into the same problem. There is an issue with the current version of the iFrame widget.
I already filed an issue report to support. If you can't wait for the official solution, you can download a fix (look at forum post https://forum.mendix.com/questions/6464/IFrame-widget-in-Mendix-5)
Have you downloaded the iFrame widget from the appstore? It should enable you to do this. Make sure you download the version for Mx5. Also, some public websites cannot be embedded via iFrame, not sure if that is the case for the website you mention.
I have downloaded IFrame from the appstore but i dont know how to use it in a corect way.
Per the documentation, you'll need an entity with 1 or 2 attributes. One will contain the URL to embed (this one is required), and one will contain a URL prefix ( this one is not required ). Then create a dataview of the entity and place the iFrame widget in the dataview. Point the widget to the appropriate attributes and it should work. Note that you'll need an instance of the object in order for this to work, i.e. if you open an empty dataview without an object in it, Mendix won't have anything to display.
Dox pasted below:
Properties Height Height of the viewer in pixels. Use '0' to determine the width automatically. This will use any remaining space.
Scroll [auto, always, never] Defines whether to scroll when the contents exceed the IFrame size. Always will show scrollbars even if not strictly needed. Default: auto
Setting this property does not guarantee the expected behavior, the styling of the website displayed has much influence on the scrolling behavior of the iframe.
URL Attribute The attribute that returns the URL of the IFrame.
URL prefix Optional URL prefix. The attribute value is prefixed with this value
Target page contains dataview "dataview1" that needs an object. Therefore, it cannot be used directly. Use a microflow data source on the dataview or call a microflow that retrieves an object and then shows the page instead.
This is the error.
I placed an object. Url (Entity) caller of the page.
I am using a microflow to get the URL form the database and then use the IFrame to show the page. Now it is loading but it keeps loading. I found in the forum some questions about loading.