When posting to a Facebook "Page" account for your business there are some cool tools to add captions to your video. Simply follow these steps:
1) After the video is processed and uploaded you Facebook, click the video and it will pop op like this.
2) On the top R of the video click the image with 3 dots "..."
3) From the dropdown menu, click "EDIT"
4) Click the tab "CAPTIONS"
5) Click the "EDIT CAPTIONS" box.
6) Follow the timeline, check the auto text and fix any errors or manually add your own text. Then click the button "SAVE TO VIDEO"