Summit members should be able to get Stretch seating at the time of booking by selecting a specific seat (rows 1-4). Don't accept the default general "aisle" or "window" option or you won't get it.

Ascent members can either pay for Stretch seating (15 or 25 depending on their fare) or for free at the gate, if available.

I know that gate agents are supposed to page Ascent members and offer it, but that generally doesn't happen. So, Ascent members, ask if Stretch seating is available when you get to your gate.