I am trying to use Azure AD Auth in Azure AD B2C using the /common Azure AD enpoint. According to How to sign in any Azure Active Directory (AD) user using the multi-tenant application pattern, there needs to be a mechanism to either:

Allow multiple issuers, or
Specify a list of issuers b2c can validate against.

I do not see any possible setting in the custom policies XML that would allow this. I was able to get Azure AD auth created successfully in my TrustFrameworkExtensions.xml file, just not multi-tenant now.

In order to support multi-tenant Azure AD, you'll need to configure your ClaimsProvider in the custom policy with different values.

Use the values below, making sure you replace with client_id and IdTokenAudience.

<Item Key="DiscoverMetadataByTokenIssuer">true</Item>
<Item Key="ValidTokenIssuerPrefixes">https://sts.windows.net/</Item>
<Item Key="authorization_endpoint">https://login.microsoftonline.com/common/oauth2/authorize</Item>
<Item Key="client_id">df5b2515-a8d2-4d91-ab4f-eac6e1e416c2</Item>
<Item Key="BearerTokenTransmissionMethod">AuthorizationHeader</Item>
<Item Key="scope">openid</Item>
<Item Key="UsePolicyInRedirectUri">false</Item>
<Item Key="HttpBinding">POST</Item>
<Item Key="response_types">id_token</Item>
<Item Key="IdTokenAudience">df5b2515-a8d2-4d91-ab4f-eac6e1e416c2</Item>

CAUTION: This functionality isn't officially even in preview yet, so use with caution. Keep monitoring the official "Sign in by using Azure AD accounts" documentation to see when this is fully documented and supported.

EDIT:

This functionality is now in preview, see the official documentation for this: Allow users to sign in to a multi-tenant Azure AD identity provider using custom policies