You need to activate the WordPrezi plugin.

For a prezi with a url of https://prezi.com/view/5N3TTfbIsN4sUGQ5N0pi/

you need to use this shortcode

[prezi url="http://prezi.com/view/5N3TTfbIsN4sUGQ5N0pi/embed" width="550" height="400" zoom_freely="N" ]

You can generated it here: WordPrezi · Embed Prezi into WordPress using the WordPress.org option. 13 Jun 2023, the site seems to be down at the moment, you can copy the above shortcode and change the url.