Click the share button in the top left then Embed:

Then you need to get the link the src attribute between the quotes,

So in the below example, that is  

https://miro.com/app/live-embed/uXjVOqoaTqI=/?moveToViewport=-14628,-7760,29253,15311

The final URL should not have the quotes either.

From there, you can add that link as an embed in Tettra:

Hit enter, then you should have a Miro embed. Miro blocks the board being visible on initial load, so viewers will have to click the  See the board button to view the board.