I ended up solving the problem. Turns out that because we still have On-Prem shared accounts that we can’t yet migrate for various reasons that the solution was to enter the O365 credentials (
firstname.lastname@example.org) the first time that the box appeared. And then on the second time that the box appeared we entered the
DOMAIN\user version of the credentials there for satisfying both the O365 requirement as well as the On-Prem server.