Originally Posted by
flyerCO
OP got it if you read their last post. Also prefered seat is supposed to be a ST E+ benefit across airlines. F upgrades are a DL provided benefit, unlike seating which is a ST provided benefit.
The issue is that usually DL doesn't pass on the DL FF # to other airlines for bookings (to KE, DL never ever does so, for example). So, the other airline has no idea that you are ST E+. Simply need to go to their site and add it, if possible, or otherwise call them and add it. When the OP did so here and had CZ add their DL SM # (which until then likely wasn't on the res with CZ), then the ST E+ seat selection was available (adding the passport info is immaterial, so not sure why was a big deal made of that).