Power Apps Portals support multiple authentication methods. For external accounts, managed through Contact records in D365, the process of registration used to require the user to provide a separate Username and Email address. Although this is still the default scenario it is now possible to require the user to provide only their email address for sign-in.

To modify the registration process, change the Authentication/Registration/LocalLoginByEmail Site Setting from false to true. No more having the use some crafty JQuery to populate a (hidden) Username field with the content of an email address.