From what I see (based on a very quick look), Behance’s API is a bit more developed than Dribbble’s.
For both, you personally need to register as a developer. But the respective users of your application do not have to do that.
Rather, for Dribbble, you’re using it as an authentication provider. (Meaning that you’re using a function similar to Facebook login, where they’re pairing their Dribbble account with the Bubble user account). Only in that case are you able to pull their account details. (Conceptually, this makes sense because I believe Dribbble is a closed ecosystem).
For Behance, it looks like profiles are public, so all you need to have is their user ID and you can pull projects by calling that endpoint.
Dribbble uses OAUTH2 so bubble should have no trouble accessing via the API Connector. They will then just have to authorise access to your app with the userid/password.
Bear in mind that you can’t (with native bubble) easily have more than one OAUTH2 connection at once, as it creates users in the Bubble back end.