You can't pick the seats in Manage my booking,there are 3 ways to book seats on S7:
-At the time of purchase (either pay the fee,or complimentary if you are S7 (not OW) elite logged in from your account;
-During OLCI
ay a fee if you are on "Basic" fare (otherwise the system will assign a random seat),OW Ruby and above can choose any seats on any fare for free,including bulkhead and emergency exits;
-Email
[email protected] with your booking ref and desired seat number,they will book you a seat and reply within a few hours.