How do I get a redirect URI?

Go to https://www.dropbox.com/developers/apps, select your app, and then you will find a box where you can add a redirect uri.

Thanks Stephen, I see where to input it but I'm not sure what it should be exactly.

If you're integrating with Dropbox with a mobile app or a desktop app, you don't need to do add an explicit url here. What you do need to do is to register your app at the OS level to handle a particular url scheme (db-<YOUR APP KEY>), which will handle the auth redirect.

If you're integrating with Dropbox with a web app, this will be a web url that your server handles.