The issue is due to Discourse’s security filters removing user-input scripts. Johani suggests using a theme component to make it work, providing code to add to the theme component and instructions on how to whitelist the AddEvent script. Grayden_Shand confirms the solution works, and later shares a custom theme component (https://github.com/graydenshand/discourse_upcoming_addevent) that hijacks the DiscourseCalendar plugin’s “Upcoming Events” page to display the AddEvent calendar, requiring a customized AddEvent script for re-renders between page transitions.
The cause here is that Discourse filters user input in posts as a security measure. You wouldn’t want random users to run scripts on your site, so we filter them out.
You can use a theme component to make this work. If you add something like this in a new theme component - under the header tab - and then add it to your active theme, you’ll be able to use that embed.
Instead of embedding the calendar in a post, which is too narrow to display the desktop version of the embed, this theme component hijacks the DiscourseCalendar plugin’s “Upcoming Events” page.