https://poly.google.com/view/6Vwdwdvy6oQ"

shortcode
should show something like this:

WordPress removes iframe html tags because of security reasons.
Iframe shortcode is the replacement of the iframe html tag and accepts the same params as iframe html tag does.
You may use iframe shortcode to embed content from YouTube, Vimeo, Google Maps or from any external page.

If you need to embed content from YouTube, Vimeo, SlideShare, SoundCloud, Twitter via direct link, you may use https://poly.google.com/view/6Vwdwdvy6oQ shortcode.