This is how you can add a YouTube video in your post:
Find a youtube video, for example:
https://www.youtube.com/watch?v=hQexRTGCeqIThen in your post options click on the Insert Video button as displayed by the blue arrow here:
Then enter the link in the white square and your video is displayed:
One other method is putting the [youtube] and [/youtube] tags around the link.