This article explains how to create a service account for migrating Dropbox drives to onedrive, SharePoint or Google Drive
Prerequisites: You must be logged into dropbox.com with an administrator account.
Navigate to https://www.dropbox.com/developers/apps
Click on Create App
Choose an API and select Dropbox Business API
Choose the type of access level : Select Team member file access
Once the application is created, edit it and generate an access token.