Hello EasyPME,

To load an iframe in backend you have to create one text field and give HTML widget to that field in XML. Now whenever you want to load an iframe on that field you need to render template using ir.ui.view into backend.you can refer below line of code to render template.

self.env['ir.ui.view']._render_template(View template id, values)

Also, if you want to load iframe on that field then you have to extend FormRenderer Js class to set your custom iframe on that field.