First, create an app from dropbox. To create, go here.
1. Select Scoped access.
2. Select Full Dropbox access.
3. Enter the App Name. And click on Create app button.
After creating the app, insert your site URL in Chooser / Saver / Embedder domains field and click on Add button. Also, make sure without HTTP or HTTPS protocol, just plain domain (ex: example.com).
On that page, you will see the App key.
Store or copy the key and use it where needed.