Exit rows on TG

Recently arrived on Thai.         475 seemed ok and almost last minute.   Checked in 24 before and almost everything booked seatwise so I managed to get an upstairs seat.  

Turned up early and enquired about exit rows.   I was told to go to the ticket office.  So I did and the reply was that the flight was full.      I asked how to do it in the future and told to arrive early.     I then said that I got to terminal 2 three hours before.     

So I got on the flight and made my way to the few rows upstairs at the back.    Flight full up ??   Hahaha I am not complaining as I and a few others got a an empty row of 4 to sleep on.    Nobody in the exit rows

The question is how to get a boarding pass with an exit row on it. 

Oh oh and I nearly forgot.  It left 90 mins late no explanation no apology food ok and good atmosphere upstairs.