When adding an IFrame to an entity form, Dynamics CRM requires that a default URL be defined. When we do not know the URL at design time, we commonly would just set the URL to the string about:blank. When we type this on a browser, this basically tells the browser to load an empty HTML page which is the ideal approach when the URL should be defined dynamically.
The problem is that when we attempt to do this on Dynamics CRM, we get this error:
Invalid Protocol. Only HTTP, HTTPS, FTP protocols are allowed on this field.
This new constraint was added on the Update Rollup 12 for Microsoft Dynamics CRM. A way to overcome this is to just to set the URL to this:
This allows the validation of the form to pass, and it set the URL to a blank page as well.