First copy the embed code on the YouTube page by clicking Share and then Embed. It should look like this: <iframe width="640" height="480" src="https://www.youtube.com/embed/wAxo6pswlU0" frameborder="0" allowfullscreen></iframe>
Then you can paste that code into the text box here, highlight it and then click the Insert Video Embed Code button, which looks like a play button on a remote control. Your code should look like this:
Code:
<iframe width="853" height="480" src="https://www.youtube.com/embed/L3-_aurNiic" frameborder="0" allowfullscreen></iframe>
And if you preview you should see the video embedded like this:
https://youtu.be/L3-_aurNiic