The integrated sunglass leash comes with all Guide's Choice and Highwater sunglasses. It is also compatible with the Challis, Colson, Captain's Choice, Transfer, and Transfer XL models.
To install your leash, you need to remove the current temple pads. Push on the pads and pull out. Keep original temple pads for future use. Start at the outside of the temple arm, line up the pad in the same direction as the opening. Slide the leash pad into the tip of the temple first, then push the remaining pad into place. You can also take a look at the video tutorial available here.