MH charges a fee for seat selection for the economy cabin for most fare classes prior to OLCI. OWE can select standard seats (non-bulkhead, non exit row, etc) for free at the time of booking. Since this is a BA codeshare, different rules may apply.

FWIW, even for OWE, seat selection is buggy on the MH website. You may have more luck on the MH app. Seat selection becomes free 72 hours prior to the flight.