code: public void doStart(HttpServletRequest request, HttpServletResponse response)
              throws IOException, ServletException {
          if (!common.checkPost(request, response)) return;
          User user = common.requireLoggedInUser(request, response);
          if (user == null) return;
  
          // Start the authorization process with Dropbox.
          DbxWebAuth.Request authRequest = DbxWebAuth.newRequestBuilder()
              // After we redirect the user to the Dropbox website for authorization,
              // Dropbox will redirect them back here.
              .withRedirectUri(getRedirectUri(request), getSessionStore(request))
              .build();
          String authorizeUrl = getWebAuth(request).authorize(authRequest);
  
          // Redirect the user to the Dropbox website so they can approve our application.
          // The Dropbox website will send them back to /dropbox-auth-finish when they're done.
          response.sendRedirect(authorizeUrl);
      }
docs: Start the process of getting a Dropbox API access token for the user's Dropbox account.