You cannot add a closed-captioning file to an online video if you are not the owner.
If you view a video in the Canvas by Instructure app, iOS devices can display captions created with WebVTT files. If you cannot view video captions in an iOS device, the caption type is not supported.
Create Caption Files
Amara is the same service referenced in our lesson about creating captions using the subtitle creation tool. The difference is that with YouTube and other third-party sites, your video is already uploaded to the Internet, so to create captions, all you need is the URL to the video.