Hello Fahad Ansari,
Welcome to microsoft Q&A, Thankyou for posting your query here.
For multi-container apps in Azure Web App where the UI (azure portal) does not support enabling authentication directly, you typically need to handle authentication outside of the App Service's built-in features.
You can use OAuth, OpenID Connect, or other authentication protocols and integrate with identity providers(like Azure Active Directory).
Consider building authentication within your application, using a reverse proxy, or an external service like Azure AD B2C.
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