trying to Auth in swift using generated access token

The API was designed with the intention that each user would link their own Dropbox account, in order to interact with their own files. However, it is technically possible to connect to just one account. The SDKs don't offer explicit support for it and we don't recommend doing so, for various technical and security reasons. 

Specifically, client-side apps, such as iOS apps written in Swift, fundamentally can't keep secrets. So, any malicious user of your app could extract the access token from the app, and use it directly, bypassing any access controls you may have tried to enforce.

However if you did want to go this route, instead of kicking off the authorization flow, you would manually use an existing access token for your account and app. (Just be careful not to revoke it, e.g. via https://www.dropbox.com/account/security .)