Either measure the keyhole slots and drill into the wall, probably should use a small anchor...
Or, rig up something out of wood that you can use as the adapter plate between your bracket and the speaker... 1x2 piece of wood, or maybe some good quality 1/2" plywood (not sheathing) but rig it to fit the keyholes of the speaker, and the attachment points of your bracket. Paint it black if you are concerned about visibility.