Application token key

guymayor Здравко is correct; the Dropbox API doesn't offer a way to upload files based only on the app key and secret. The app key and secret only identify the app itself. Making changes to an account, such as uploading files, requires authorization to access that account.

The Dropbox API doesn't offer any sort of "app account" or "app storage" where you can upload files outside of any specific user account, but I'll pass this along as a feature request. I can't promise if or when that might be implemented though. 

To get an access token for a specific user account for the Dropbox API you need to send the user through the OAuth app authorization flow.

Likewise, to get an access token for an entire Dropbox Business team for the Dropbox Business API you need to send a team admin through the OAuth app authorization flow. If the Dropbox Business API app is registered for the "team member file access" permission, you can then use the resulting access token to access any member's account via the "member file access" feature documented here. The individual member doesn't need to additionally authorize the app, as it's been authorized for the entire team by a team admin.

Finally, an actual Dropbox API access token does not expire by itself, but it can be revoked by the user or app at any time. Also, note that "access tokens" are not the same as "authorization codes". Authorization codes are short-lived, single-use codes that can be exchanged for an access token.