Acionando a criação de conta/login em serviço externo quando um usuário faz login no discourse

My answer assumes you are using Discourse as your identity provider (with its login/signup UIs), and wants to keep it that way.

On the Discourse side, enabling it is as simple as

However, you mentioned you’re building a plugin.

If you build “a path on the server” in a new controller action in a Discourse plugin, you can get the user from the session, call 3rd parties and return the JWT to your client.