code: public String start() {
          DbxWebAuth.Request request = DbxWebAuth.newRequestBuilder()
              .withNoRedirect()
              .build();
          return auth.authorize(request);
      }
docs: Start authorization.  Returns a "authorization URL" on the Dropbox website that gives the
  lets the user grant your app access to their Dropbox account.
  
  <p>
  If they choose to grant access, they will be shown an "authorization code", which they
  need to copy/paste back into your app, at which point you can call {@link #finish} to get an
  access token.
  </p>