If you want to add a video, click on the video icon and paste the video URL in there. After you click "Insert", it will show up in the body of the post!
Please note that due to our current post editor limitation, we can only accept videos from YouTube and Vimeo. We will switch to an improved editor as soon as possible and will have more options available. For now, if you want to add a video to your post, you first need to upload it to either YouTube or Vimeo and then use a video link from that service.