Please check the following Apple Support article on using the third-party cloud apps in the Files app: https://support.apple.com/en-us/102238

Follow these steps to save your files to a specific location by default:

iOS

1. For third-party storage providers like Dropbox, Box, and so on, install their app on your device and log in. For iCloud, you can skip this step.
2. Open the Settings app
3. Scroll down until you find MindNode for iOS
4. Locate the entry “Document Storage”

5. Select "On My iPad/iPhone" or the cloud storage provider of your liking. 

Note: Third-party providers like for example Dropbox only get displayed once you download their app from the App Store. 

macOS

MindNode for Mac remembers the last location you saved your documents to and creates new documents in this location.