Thanks! Based on the post you linked to I was able to determine that this forum’s domain whitelist is subdomain sensitive. For OpenProcessing you must use https://www.openprocessing/ and for p5js.org you must use https://editor.p5js.org/. So beware the generated <iframe /> tags from those sites because they actually use different urls.
Here’s a working example:
<iframe src="https://www.openprocessing.org/sketch/1116549/embed/" width="100%" height="510" allowfullscreen frameborder="0"
marginwidth="0" marginheight="0"></iframe>