You have find both the Postman environment and the Postman collection. You just need to register your own application with Dropbox and to have your own consumer key to get started. Put the values inside the Postman environment. Make sure you configure the Permissions as well.

Please note that you the Postman collection does not include the following configuration:

Set the following URLs:

https://www.dropbox.com/oauth2/authorize

https://api.dropboxapi.com/oauth2/token