No there is no place to enter seat number selection. Only the berth types(Lower,Middle,Upper, Side Lower, Side Upper) preference may be mentioned.
Never go for 1st Seat(in Sleeper class & 2nd AC and 3rd AC) because in case of Sleeper class the toilets will stink a lot. In case of 2nd & 3rd AC, door opening/closing will be too much of disturbance. There is also higher risk of theft in these beginning areas.
So the most secured area will be the berths exactly in the middle of coach.
But if you still want to then yes booking seats usually goes serially from 1st seat to 72nd(in SL) unless accommodation/preference is not satisfied. But remember initial block of berths(9 or so) may be reserved( ladies etc).
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