We have a scenario where we are making use of the Alternate Login ID for external users so they do not need to know the email we have generated for them in our domain.
When entering the Alternate Login ID at https://passwordreset.microsoftonline.com/ it responds with an error "Microsoft Accounts or personal email addresses don't work here. Make sure that you enter your work or school account (e.g. email address removed for privacy reasons). If you want to reset a Microsoft Account password, click here." which makes sense since it has no way of knowing which organisational domain it may relate to.
I know you can customise your portal by appending /?whr=yourdomain.com to the end which would then give it an idea of the domain the account may belong to however the same error appears.