Hide asking confirmation on login

That's correct, access tokens are user-specific. It sounds like you mean that your app doesn't have its own user model, and is relying on the Dropbox app authorization flow as authentication. First, it's worth noting that OAuth 2 is for authorization, not authentication, e.g., see these blog posts. Anyway, if you do use this for that anyway, to avoid unnecessarily putting the user through this flow, you can store the necessary data, e.g., the access token, in their cookies or local storage. If and only if you don't find that, you can send them the flow. Also, check out the documentation for the force_reapprove parameter on /oauth2/authorize. By default, Dropbox will automatically redirect the user through the flow when possible.