I have added an Iframe in a custom entity to display a custom aspx page (say http://localhost:55808/WebForm1.aspx) . I have unchecked the Restrict cross-frame scripting, where supported for that particular iframe. After saving and publishing, I have got the result as the below screenshot.
In the above image, the highlighted box is the Iframe that I have added which is shown as blank and non-editable when a new form is loaded.
Awaiting your suggestions and responses.