So you have got some videos on other webpages which you wish to share with your users within the channel. What do you do? You can't download the video from the site so you generate an embed code from that webpage.
To paste the code you copied, click on Upload Content > Embed External Video. Here you can paste the code in the Embed tab, fill in the title and description and hit save. The embedded video can be seen within the community you saved the embed in or in the Manage Content page. Remember to first approve the video, so that all your users too can view it.