Cognito - How can I access to federated IdP's resource server?

[wataruoguchi]

This question may not related to the JS library but Cognito in general.

There's a resource server endpoint I want to access. That requires federated ID provider's access_token. However, through Cognito, we can only access to Cognito's access_token.
How can I access to federated IdP's resource server?

Can I retrieve IdP's access_token somehow? Or does Cognito provides an endpoint?

Thanks!