As above - there has to be reward availability in the class you want, then it’s just a case of ringing the executive club phone line and paying the extra avios, the extra taxes and fees and the £35 admin fee per person.
if there’s none currently available, join seatspy and set an alert to email you if and when one comes up - I’ve done this twice now, on outbound and inbound legs on a Malidives flight, in club both ways.