Thank you for posting this in Microsoft Q&A.
I understand you want get continuation_token on the first step from Native authentication API.
We will get challenge_type = redirect
If an app can't support a required authentication method by Microsoft Entra, a fallback to the web-based authentication flow is needed. In this scenario, Microsoft Entra informs the app by returning a redirect challenge type in its response.
Microsoft Entra's native authentication API supports sign-up and sign-in for two authentication methods:
1.Email with password, which supports sign-up and sign-in with an email and password, and self-service password reset (SSPR).
2.Email one-time passcode, which supports sign-up and sign-in with email one-time passcode.
The value is expected to oob redirect for email one-time passcode and oob password redirect for email with password authentication method.
Could you please confirm which authentication method you are using: Email with password or Email one-time passcode? Also, can you ensure that the same authentication method is used for both sign-up and sign-in?
If you are using Email with password The user that you use to sign in has to be created via an email with password user flow. For a given authentication method, the challenge type values an app sends to Microsoft Entra during sign-up flow are same to when the app signs in. For example, the email with password authentication method uses oob, password and redirect challenge type values for both sign-up and sign-in flows.
Thanks,
Navya