Authenticate on-premise users without verifying the Domain

[Palayathar]

Dear Experts

This is the scenario. Our client is a part of large organisation that comprises many companies. All users in this company are all using the same namespace for on-premise access. e.g. rootdomain.com

Users from our client and their parent organisation uses the following credentials to login to on-premise resources <username>@rootdomain.com. However our client do not have the control of the rootdomain.com and they will not be able to verify the ownership.

Now, they have procured Office 365 services [Power BI] and have a tenant say, client.onmicrosoft.com. 

They are asking; if their users can use their existing on-premise credentials to authenticate against Azure AD. My understanding is that; it is not possible to do this without verifying the domain [rootdomain.com] and without using AAD connect 

Am I correct?

If not, is there any way to authenticate to Azure AD using a third party authentication providers by using some apps in Azure?

Thanks in advance

[#2 · Accepted solution]

The only way to use their on-premises credentials is to verify the domain, that includes any auth method that uses attributes other than the UPN as well. Perhaps they can verify a subdomain instead of the root domain?

[#3]

@Vasil Michev  Thanks.