Dropbox API to create a file, write in it via stream and then close the file.

To upload a file using the API, you can use the /2/files/upload endpoint:

https://www.dropbox.com/developers/documentation/http/documentation#files-upload

That is a "Content-upload" style endpoint, so it expects the file contents in the request body, as documented in the "Request and response formats" section:

https://www.dropbox.com/developers/documentation/http/documentation#formats

There's an example of uploading from PHP using curl here:

https://stackoverflow.com/documentation/dropbox-api/409/uploading-a-file/1354/uploading-a-file-via-curl-in-php#t=20170419174709470892

Exactly how you access the data on your local client is up to you. The API just requires that you pass up all of the file data in the request body.