Azure B2C client credentials grant

It is now possible to use OAuth2 Client Credentials grant type with Azure ADB2.

Although the OAuth 2.0 client credentials grant flow is not currently directly supported by the Azure AD B2C authentication service, you can set up client credential flow using Azure AD and the Microsoft identity platform /token endpoint for an application in your Azure AD B2C tenant. An Azure AD B2C tenant shares some functionality with Azure AD enterprise tenants

Here is a sample curl request:

    curl --location --request POST 'https://login.microsoftonline.com/{client-id-of-app-registered-in-b2c}/oauth2/v2.0/token' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--header 'Cookie: x-ms-gateway-slice=prod; stsservicecookie=ests; fpc=AmqL7OwikMNGgdpvjdkb0OLnguDtAQAAABl14NYOAAAAd_wwNgEAAABCeeDWDgAAAA' \
--data-urlencode 'grant_type=client_credentials' \
--data-urlencode 'client_secret={secret-of-app}' \
--data-urlencode 'client_id={client-id-of-app-registered-in-b2c}' \
--data-urlencode 'scope=https://graph.microsoft.com/.default'

Note that the parameters are encoded in the body.

See this link that describes the Azure Active Directory B2C limitations. The quick-start you referenced is using the client credentials grant, which is not yet supported in Azure AD B2C.

Under the section Daemons / server-side applications it reads:

"Applications that contain long-running processes or that operate without the presence of a user also need a way to access secured resources, such as Web APIs. These applications can authenticate and get tokens by using the application's identity (rather than a consumer's delegated identity) in the OAuth 2.0 client credentials flow. This flow is not yet available in Azure AD B2C, so for now, applications can get tokens only after an interactive consumer sign-in flow has occurred."

I believe this feature (oauth client credentials grant type support) is on the B2C roadmap and when it is released, the steps in that quick start should work.

It is a well documented limitation, I have created user voice request on the feedback portal.

You can vote it and wait for the Development Team to implement it.