For RML for Dropbox to query your Dropbox account, you need to create at least one App and get its Key and Secret. Here are the steps to create a Dropbox app:
1. Login to your Dropbox account online as you would normally do
2. Access your App Console and click on Create app
3. Fill the following settings as shown in screenshot below. Choose Dropbox API, Full Dropbox and provide a unique name for your app. Then, click on Create app. Note: App Folder option is not fully supported yet, but it will be shortly.
4. In this next screen, first click to Enable additional users. Then, authorize Redirect URIs for your sites by adding the base URL of each site with the added path /ocsrmlfordropboxcallback. For example, if your site base URL is blog.wordpress.com/, then you should add https://blog.wordpress.com/ocsrmlfordropboxcallback.
Note: HTTPS protocol is a requirement of Dropbox, although it allows HTTP protocol for localhost domain. If your site is not yet setup for HTTPS, contact your hosting provider. You can setup HTTPS for your site for free via the Let’s Encrypt Authority service.
5. That’s it! Your App is now ready for use. Grab your app key and secret from this screen and paste them in the appropriate text fields labeled App Key and App Secret of your Dropbox Remote Media Library in WordPress (Media > Remote Libraries).