Here is the method I found on another forum

Type the word "file" in the text editor
Highlight the word "file" and press the link button
Change the type to file and paste the embed url
Toggle to the HTML Source code (<>)
Find the word file at the end of the code and delete it
Toggle back to the Editor (<>)
Save

I haven't tried it in a forum but it works in the add a text page assignment activity so it should work in a forum.

Just make sure you use the plain embed code not ones for specific sites.