To post a video or, for that matter, a photo, first the video or photo must be uploaded to a sharing site like YouTube for videos or Imgur for photos. For videos I don't think you can embed to this site, but you can include a link in your post to the video host address.
Such as. A wind tower transport trailer I designed When you insert the link you will see part of the text highlighted in blue. You edit this as the description you will see in the post.
For photos there is another post here by PG409 that describes how to embed pictures.