Serverside Auth Flow for Third Party Users

What is the best flow for machine-to-machine auth initiated by a third party user?

I’ve looked at many flows from Okta’s documentation and am wondering which flow would be best to authenticate and authorize a third party user.

So far it seems the authn followed by authorize calls are the best way for the third party to generate id_token and access_token which can then be used to hit our APIs.

I have searched high and low for documentation or examples of authenticating a user from their server but everything I’ve read requires a hosted web solution which requires a redirect_uri…