Find the Tweet you want to post and click or right-click > copy URL of the "time ago" it was posted.
Screenshot 2015-02-27 00.34.19.png
The URL you captured or that you will copy looks like this.
Screenshot 2015-02-27 00.37.36.png

Paste the URL and the board will auto-detect and interpret it as a Tweet:

https://twitter.com/TheOnion/status/571090702005891072