To get the online link for a piece, click on the title of the piece to reach the "Create Marketing Piece | Edit" or "Create Marketing Piece | Distribute" (depending on if the piece has yet been sent or not).
Scroll to the bottom and click Create Public Link to generate the unique online link for your marketing piece. You can copy the URL there in that area after generating it, or click on the link to view the piece in your browser and copy the URL from that page.