Thank you for reaching out to the community forum!
For your query, I understand that you are trying to integrate the third-party directory users with Microsoft Entra ID.
To achieve you are ask you can use Microsoft Entra External ID to securely interact with users outside of your organization. This allows external users to use their own identities to sign in, which is ideal for collaborating with partners, distributors, suppliers, or vendors.
Also, you can use the B2B Collaboration which invite external users to sign in to your Microsoft Entra organization using their own credentials.
The other side if you want to federation with any organization with Entra ID the identity provider (IdP) supports the SAML 2.0 or WS-Fed protocol for additional reading Federation with SAML/WS-Fed identity providers for guest users.
Deploy Active Directory Federation Services in Azure
Hope this helps. Do let us know if you any further queries.
Thanks,
Akhilesh.
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