As you can tell Wayne, I’m not that familiar with this topics. But I just ran a test adding the iFrame code that I get from Forms system I’m using (JotForm) and I was able to publish it and it works correctly:
https://pruebas.psicoevaluaciones.com/12345-2/
I just copied the code in the ‘Text’ editor of WordPress’ Post.
This is what I’m trying to achieve, but it doesn’t seem to work when I send it through Postie (exactly same code).