Azure App Service Authentication. This article describes how app service helps simplify authentication. By default, app service provides authentication but does not restrict authorized access to your site content and apis.
Obtain the client id of the azure ad application that the app service is using for authentication. Its worth checking the manifest of both client and api app registrations on azure ad. From the client app blade click on manifest.
In the upcoming properties pane look for authentication/authorization option like below,
It is possible to enable basic authentication for azure web apps with some settings in the applicationhost.xdt. Azure app service includes a turnkey authentication solution, under the authentication/authorization settings blade. One of the goals of azure app service authentication / authorization is to make it very easy to add auth to your app service apps (which is why we often refer to it as easy auth). Turn the authentication on and use azure active directory as the authentication provider.