NEW Outlook app: how to add an IMAP account with user name different from email address?

Ergh - now Outlook 365 is trying to get me to switch to Outlook New - which immediately failed after it imported my IMAP settings.

UPDATE: After I told Outlook (new) to go back to Outlook 365, it hasn't asked me to migrate again. So - my Office 365 workaround is still in place for now.

2nd UPDATE: However - new Outlook is force loading. If I uninstall Outlook (new) - reboot - verify that everything is clean - then start Office 365 - Outlook (new) is automatically loaded.

Yes - I think this is exactly what's happening. Some IMAP providers can apparently parse this - the ISP I'm using does not.

I'm working with them to see if they can discard the "@domain.com" part of my forced Outlook (new) IMAP login - but it's not looking promising.

It may also be that Outlook new is sending the "user @ domain.com" first, then "user" as it logs in - but some IMAP providers disconnect after the first failed attempt. I can't get enough of a trace with my ISP to tell if that's the case.

Your provider is working with the email address login, I'm afraid. Behind the scenes.

The screen shot you provided shows that Outlook new is sending the email address as a login.

My email provider does not autoparse the email address as a login. Yours does, apparently.

Thanks, though - you've been very patient with this.

it is possible your "email looking" username can be used to setup the account which will eventually work with the intended address.

Example , i can add my account using my ...@onmicrosoft.com address and still eventually use my proper email address (actual domain)

Well, there you are wrong. The username and the email address are different but I'm not showing that info. I use WPX Hosting for my IMAP emails and they do not use same email & username credentials.