How to upload file in dropbox in each and every user using vb.net

In order for an app to upload files to any particular Dropbox account, it needs authorization from the user of that account. To request authorization from the end-user, the app should implement and use the OAuth app authorization flow. (The Dropbox API v2 .NET SDK offers a DropboxOAuth2Helper class for this. There's also an example app that uses it here.)

The result of this is an access token for that user's account, which would be in place of the 'xxxxxxx' string seen in your screenshot. Using that access token would allow the app to upload to the user's account. The app only needs to process the OAuth app authorization flow once per user. It can store and re-use each access token for each respective user.

Alternatively, if it makes sense for your scenario, you could invite the other users to a shared folder in your account, and then just upload to that shared folder via the API using an access token for your own account, and those files would automatically sync to the other members of the shared folder.