There is a launch control sequence you have to do. Im not sure what it is for SMG III but if you search you shall find it!
Personally i find launch control useless though since BMW logs everytime you use launch control and if you use it too many times, they may void your tranny warranty if something goes wrong down the line. Plus launch control only lets you rev to 2000rpm in the US so its essentially useless because you can do as you did and just brake torque up to 2000 and go. No crazy sequence to initiate or any buttons to push.
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