Update   Mar 2026 adding to a class block no longer works in WordPress. You need to use the classic editor. After adding the iframe code you can save a draft and switch back to the block editor  and convert the classic block that has ben added to a shortcode block if you wish.

Updated June 2023 Glow blogs now supports pasting the waklet embed code into the Blog editor. It will then turn into a shortcake. This supports the different layouts and settings provided by waklet: