In order to register the Social Account through API, you will need to go through an account linking path where the user will already need to exist in your tenant. As you cannot fully register a user on their behalf with the social provider, the general path should look something like this :
– Have the user created in your tenant, either by themselves in an email/pw DB or manually create them/create them through API;
– Setup Account Linking in order to handle the process;
– Use The Management API endpoint link the accounts, for which the endpoint is POST /v2/users/{id}/identities;