To add automatic captions to a video you will need to use the "Auto-Captions" tool available at the top of the "Templates" tab of the app.
Upload your video, then select the language for your subtitles. If you select No Translation, the tool will automatically detect the video’s language and generate captions in that language.
Choose the Caption Style
Now you're all set!
You can use the normal editing options to edit text or add elements to your video.