When in the editor, go to the Teaser Section.
Go to Youtube, find the video you would like to use.
Click on the Share icon.
Select the Embed tab, and select show more.
Make sure Show suggested videos when the video finishes option is not selected.
Copy the Embed code.
Go back to the web page editor, click the Source tab in the Teaser Section and paste the link in where you want it in the page.
Press Save & Preview to review, and then press Publish when you are satisfied.