Azure-sdk-for-net: [DOC] Add Cognitive Services requires custom subdomain for TokenCred to docs

[jongio]

TokenCredential requires that the cognitive service be created with a subdomain.

This should be added to docs and readmes.

https://docs.microsoft.com/en-us/azure/cognitive-services/authentication#create-a-resource-with-a-custom-subdomain

See #12777

azsdke2e

[jongio]

Looks like already there, but I missed it.

https://azuresdkdocs.blob.core.windows.net/$web/dotnet/Azure.AI.FormRecognizer/1.0.0-preview.3/api/index.html#create-formrecognizerclient-with-azure-active-directory-credential

Note that regional endpoints do not support AAD authentication. Create a custom subdomain for your resource in order to use this type of authentication.

[jongio]

Reopening with this suggestion:

I think part of it is that I didn't know there were two models.
Maybe a section in readme to talk about the two models or a link to a doc that does

[kristapratico]

It looks like the error message has been updated to be more clear when AAD auth is attempted without a custom subdomain name:
(BadRequest) Please provide a custom subdomain for token authentication, otherwise API key is required. (Please see https://aka.ms/cogsvc-authenticatewithtoken)

[jongio]

@maririos - I suggest that we call this out better in docs. It's there, but would be good to explain the difference and what each one looks like.

[maririos]

Fair point