You need to supply a https:// url for the embedded content to work.
The URL entered is automatically changed into https://
To circumvent this, you can create a https:// custom shortcut URL at http://tinyurl.com
You can use the
https://tinyurl.com/............your....URL...................
link to embed the content of your website in this system.
There is a drawback:
Some browsers may display a warning message for mixed https:// and http:// content.