Azure AD B2C integrating with corporate (Azure?) AD accounts

What you are referring to is having Azure AD in multi-tenant mode, added as an identity provider to Azure AD B2C.

From: Multi-Tenant Azure AD Auth in Azure AD B2C with Custom Policies

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: Make sure you flip the Multi-tenanted switch in the App's settings, otherwise you'll get the following error:

AADSTS70001: Application with identifier '(guid)' was not found in the directory (our company's primary domain)

Azure AD B2C does not officially support a multi-tenant Azure AD identity provider. You should vote for the feature here so it can help prioritize it and so that you can be contacted when the feature is in preview.

There is a potential workaround if you have a limited set of Azure AD tenants that you work with. You could build a page in your app where the user types in the email address. Based on the email address, you can use the domain_hint parameter when calling Azure AD B2C to redirect them directly to the right sign-in page.