At this time, it is possible to embed an iframe in CRM, but it is not possible to go through the login process within the iframe and keep your CRM users within CRM while they navigate the site.

Users should be able to login to a website/portal through an iframe embedded in D365 and continue to use that site while still working within D365. Currently, users can login to a site from an iframe, but they are brought to an external URL to navigate the site.