Use the shortcut Option-C to add a caption clip at the playhead.
You can choose between ITT (for iTunes/online) or CEA-608 (standard for broadcast/web). Final Cut Pro X 10.4.1
This update introduced Apple’s high-performance RAW codec, allowing you to edit with the flexibility of RAW data while maintaining the speed of standard ProRes. Use the shortcut Option-C to add a caption
This version requires macOS High Sierra 10.13.2 or later to function. Final Cut Pro X 10.4.1
Version 10.4.1 added native tools to create, view, and edit captions directly in the timeline.