URI HTTPS request

It looks like you're trying to access the /authorize page programmatically, but this is actually just a web page that you should be presenting to the user in a browser.

For reference, while you can certainly implement this manually, I recommend checking out the example for the SwiftyDropbox library just as a sample of how this should work:

https://github.com/dropbox/PhotoWatch

Here's the library itself:

https://github.com/dropbox/SwiftyDropbox

Also, note that the "authorization code" only applies to the "code" flow, which you're not using. 

Finally, your app only provides the client ID a.k.a. app key, which identifies the app. It should never have the user's credentials (username and password) which should only be provided by the user on the Dropbox site during the app authorization flow.