Embed Image via Url is currently unsupported by Ckeditor 5 default. That's why i do this.
Everything that could make your site partial in-secure is not recommended by Ckeditor 5. Therefore Embed Image via Url feature suffer the same destiny.
However if you like this feature so bad as i do, this might help.
This version only works on @ckeditor/ckeditor5-build-classic, but you can take the idea to do whatever you want.
Remember to add a little tweak in _rwd.css, detailed in the tutorial text file.
Enjoy!