To post a video from YouTube or Vimeo all you need to do is copy and past the web URL or Share URL of the video directly into the post and and press enter on your keybord. The code your looking for is shown in the images below. (Highlight the URL, right click - select copy. In the posting box right click and select paste then press enter.)
https://www.youtube.com/watch?v=ngElkyQ6Rhs
OR
https://youtu.be/ngElkyQ6Rhs
Demo