There is a similar but different Question here: This question + answer but the proposed solution to use API Tokens is not what i need.
I need an OAuth like interface / experience where the user is a) not providing his credentials and b) us also not required to configure an API Token.
I know of one macOS app that is doing this but i don't understand how to do this from reading the documentation. I have experience with doing OAuth1.0a to authenticate with Trello REST API. But Trello has request-authorize-access-URLs and API key + secret.
How can i implement a similar flow like shown here...?

