How to test SDK/API using OAuth2?

Question

API

I want to test several SDKs before writing my own.
But everywhere there is authorization using OAUTH2 (which requires a browser redirect, which means a real url on the Internet)
Is there any workaround? Or that you need to have a "test site" to which each SDK / API will send a token, and I will copy it with handles to myself locally and develop the application?