Note: the Reaction Recorder is included with the Magic Pass. It needs a camera and microphone, and your permission to use them.
On the website (Google Chrome recommended)

In the app, go to My Creations > Videos in the Parents' Corner, select the video and tap Reaction Recorder. On iPhone and iPad, keep the app open and the screen on until the upload is complete.
The Reaction Recorder can't be used on interactive videos or videos that include a call.