Register an identity provider is the correct configuration step to perform first when you require users to sign in with Microsoft Entra ID.

You must register or configure an identity provider in the App Service Authentication and Authorization settings before the app can redirect users to Microsoft Entra ID and validate authentication tokens. Performing Register an identity provider establishes the client identifier and secret or the platform connection that enables the sign in flow and token issuance.

Assign a custom domain name is not required to enable user sign in with Microsoft Entra ID. A custom domain is only needed if you want a branded URL or to use certificates for that domain and it does not affect the initial identity provider configuration.

Create a new app configuration setting is not the first step for enabling sign in. App settings may store values such as client ids or secrets but you must first register the identity provider so those values exist and the sign in flow can be established.

Upload a private SSL certificate is unrelated to the basic user sign in setup. Private certificates are used for securing custom domains or for client certificate authentication and they are not needed before registering the identity provider.

Enable a system assigned managed identity is used for the app to authenticate to Azure resources and not for end user sign in. A managed identity does not replace the need to register Microsoft Entra ID as an identity provider for user authentication.

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