Exchange Hybrid Configuration HCW8001 Unable to determine the Tenant Routing Domain

VasilMichev, thank you for your reply!  I just found the solution.  By default, Microsoft sets up a domain for customers, like company.onmicrosoft.com, which is automatically added to Exchange's Accepted Domains.  In order for HCW to complete, it also needs a record with "mail" in it, like company.mail.onmicrosoft.com, added to Accepted Domains.  Although both domains can be easily added to on-premises Exchange, only Microsoft support can add them to Exchange Online's Accepted Domains.  I opened a support ticket, they added the domain, and HCW completed successfully.

FWIW: I think either the Entra Cloud Sync or the HCW is supposed to automatically add the "mail" domain because, in my test environment, it was automatically created.  I suspect this is an intermittent bug that only affects some users, which would explain why the solution was hard to find.