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I understand that you have a power BI report Web app in Microsoft Entra ID (work force tenant) and you would like to create an Entra External ID tenant and the users in the Entra External ID need to access the power BI report Web app which is registered in Microsoft Entra ID. which is not a supported scenario.
Because of Microsoft Entra ID tenant that contains your employees, internal business apps, and other organizational resources. here your internal users can collaborate with external business partners and guests using B2B collaboration.
Entra External ID tenant configuration is used exclusively for apps you want to publish to consumers or business customers. It contains your app registrations and a directory of consumer or customer accounts.
If you want to give the access to guest user, you can use Microsoft Entra B2B collaboration, you can invite anyone to collaborate with your organization internal users and resources using their own work, school, or social account.
For more read Distribute Power BI content to external guest users with Microsoft Entra B2B.
If you want to use Entra External ID (CIAM) tenant, you need to register an application in Entra External ID ->Add a sign-up and sign-in flow -> Add an application to the user flow.
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Thanks,
Akhilesh V.
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