I believe that these two documents can point you in the right direction:
- Call an Identity Provider API https://auth0.com/docs/tutorials/calling-an-external-idp-api
- Identity Provider Access Tokens https://auth0.com/docs/tokens/idp
Then, you should be able to use the Github access token under user.identities[0].access_token.
Note this advice may be out of date. Originally answered by @abhishek.hingnikar in old forums: https://auth0.com/forum/t/hubspot-not-quite-oauth-connection/3644/7?u=logan