Not 100% sure how it works with codesharing, but WestJet (and majority of other Canadian airlines) does not offer free seat selection until T-24, unless your fare basis on WS includes it (or have status with WS/DL).

What this likely means is that you'll have to pay to select in advance, or select at check-in, as your WS leg will be governed by their rules. FYI, not many people pay for seat selection in advance on WS, so as long as you check-in early, you should get a decent seat. You should also be way ahead of many other passengers, as T-24 is calculated from your FLL departure.