You must be set as "Presenter" to have access to the Share an external video button. You can see if you are Presenter in the User list if there is an icon of a presentation board on the left of your name.
Click the "+" action button in the bottom left corner of the virtual classroom.
Click "Share an external video."
Paste the video URL into the "Add Video URL" box in the popup window.
Click "Share a new video"
See attached screenshots to help point you in the right direction.
