Dropbox Authentication only with code

In order to get an access token for a user's account in order to interact with the user's Dropbox account programmatically, you do need to get authorization from the user at least once. An app gets authorization from the user by sending them through the OAuth app authorization flow. If the user authorizes the app, the app will receive an access token it can use to call the API, and optionally a refresh token. The app can store and re-use that, so it doesn't need to send the user through the authorization flow again. 

Depending on what settings you use though, you may need to send the use through the authorization flow again occasionally to get a new access token. Please first read the OAuth Guide for information on the different options available and what kind of input they do or don't require from the user.