What are some general ways a website could dump access to cookies or share user passwords with permission?

I am building app and I want to be able to export user's passwords or cookies to specific websites. One way is to export cookies with an extension but I would prefer for it to be as frictionless as possible. Are there any other ways that we could do this? The user can opt-in so it doesn't need to be silent, we just want it to be easy for them. Short of typing out their password, of course.