When there's a certain exercise missing in the system, or you would like to create a custom one, you can do so through the exercise database. Read all about creating a custom activity here.
When you create your custom activity, you might want to add a video in which the exercise is visualized or explained. You can do this by uploading your video to YouTube and adding the YouTube link to the custom activity. The link you need from YouTube is a long one, and starts with:
http://youtube.com/watch?v= (then followed by the VIDEO ID).
Once you've added this video, your clients will be able to view the video when opening the exercise on the website or mobile app.