Extra legroom seat booking

Hi guys,
If I call and book an Extra Legroom seat (not an exit row one) in advance, will they be able to tell me which seats I'll be allocated should I purchase and will they then show up on my booking? Or do you simply reserve two extra legroom seats and then its pot luck at the airport which you get?
Thanks!
If I call and book an Extra Legroom seat (not an exit row one) in advance, will they be able to tell me which seats I'll be allocated should I purchase and will they then show up on my booking? Or do you simply reserve two extra legroom seats and then its pot luck at the airport which you get?
Thanks!