By default, captions are turned off for generated videos. To add captions for the video select the Design tab on the lefthand menu, then select Captions, then toggle off "Hide captions." In this Design tab, you can also change the size, style, placement, and highlight color of the captions for your video.

To edit the captions for a specific scene, select the scene in the timeline, navigate to the Design tab. You can hide captions for a specific scene, change the size, and change the placement.

These articles cover the editor in the old Hypernatural experience, which is being phased out. See What's changing with Hypernatural for the exact timeline for your plan.