1. Open Marketing > SMS > Sequences.
2. On the Abandon Cart SMS card, switch the toggle on, choose a From number (required — without it the job logs "No Phone number is selected" and nothing is sent), adjust the Message if you like, and click Save.

The text is sent by the abandoned-cart job together with the abandoned-cart email: a cart counts as abandoned when it has not been checked out and has not changed for longer than Business: Cart > Abandon Cart: Timeframe to consider abandoned (in minutes) (default 30). The customer must be identified (logged-in cart with an email) and have a phone number; the message is sent immediately to that phone, shortkeys are filled in, and the balance is charged. It requires a connected SMS account, completed A2P registration for US/Canada, and a balance above the per-message rate.