CaptionSync Audio Description requests can be placed in parallel with Captioning submissions. You can also request Audio Description for existing Captioning submissions (completed or not). Here is a high-level overview:
A Captioning submission is required as well, since we use the timing in the caption file to signal where the narration occurs in the timeline, so that the descriptions don't interrupt it.
Our How-To Tutorial explains step-by-step how to request Audio Description for your videos.