All you have to do is place the full URL of the object you are trying to embed. For example, in a reply, you could place this into the replies content box. Check out this cool video:
That was a cool video. Then where you see the YouTube link URL, when you submit the post you would see a youtube video in full.
Use <pre> tag for multiple lines of code. Once again, be sure to escape any angle brackets in the code for proper rendering.