Upload a video and let Stream generate the captions automatically (On the Options tab, set Autogenerate captions to On).
Follow the bellow steps to edit the captions:
Note: If the Download file option isn't visible, you might need to uncheck the Autogenerate captions box.
Note: The downloaded file is in .vtt format which is not a recognisable format. In order to open your file by Notepad, right-click on the file and select open with. Then choose Notepad from the application list.
Another way is to click on the Notepad icon to open the program. Then navigate to the file you downloaded and make sure you have selected ‘all files’ rather than just Text Document otherwise you won't be able to see the caption file.