How do I add an OIDC identity provider to my new Entra External Id (CIAM) tenant

Question

I have a regular Entra tenant (described now as workforce ?). I now also have an external Id for customers tenant. In the past using B2C, I was able to create a custom policy that allowed a user to sign into b2c registered applications using a federated account, where that account existed in an Entra tenant. I am trying to do the same thing with the new entra external Id for customer solution. I cant find a way to an OIDC azure tenant however. Is this possible?

Thanks