There's a couple different ways to do this. Generally if you are looking to have one piece of text or HTML code update dynamically based on the content of another file you would use a Server Side Include (SSI). Where ever you insert the SSI the server will drop in the content of another file. The content could be a URL of a YouTube video if you like and the SSI could be located in a line of code for embedding a YouTube video.
To do this go to Insert > Server-Side Include.
Abraham