According to the current documentation, it’s not possible to directly use Azure AD (Entra ID) as an IDP in Entra External ID for corporate users. However, i found a workaround that can achieve a similar result.

You can leverage Azure AD B2C as an OIDC provider within Entra External ID. The flow would look like this:

Entra External ID → Azure AD B2C → Corporate Active Directory → Entra External ID

In this setup, corporate users authenticate through their usual Azure AD credentials, while External ID handles the authorization and user management on your side. This allows you to maintain a familiar login experience for corporate users even though direct IDP support isn’t available yet.
Looks tricky...