How to send trade in

My OnePlus 13 got delivered along with my watch. I made the order with a trade in. But the package it came in has no information about how to send the trade in back. From what I remember when I traded in my 11 for the 12, there were at least instructions? Do I need to call? I would like to send the 12 back and get reimbursed for the trade in value quoted.