Thanks for the demo, Rumen - it's good to know if should work. Now all I have to do is figure out where I've gone wrong!
In my layout, at the top I have this registration:
<%@ Register TagPrefix="telerik" Namespace="Telerik.SharePoint.FieldEditor" Assembly="RadEditorSharePoint, Version=18.104.22.168, culture=neutral, PublicKeyToken=1f131a624888eeed" %>
Then in the layout HTML, the field looks like this:
And the HTML I'm trying to insert looks like this:
It shows in the design view of the control, but the iframe gets stripped out on saving, leaving only the surrounding div. Can you see anything wrong?