Through the TMSP.
AUTOTHROTTLE MODES
To allow autothrottle engagement, the A/T switch must be at ARM. The autothrottle can be engaged either by manual selection of the EPR or SPEED switchlights, or automatically through selection of certain pitch modes.
When engaged manually two autothrottle modes are available, EPR and SPD.
When EPR mode is engaged the autothrottle will open the thrust levers to the maximum limit currently selected on the Thrust Mode Select Panel. EPR mode cannot be engaged on the ground unless TO is indicated on the Thrust Mode Select Panel. At 80 knots on take-off the autothrottle enters an unpowered condition known as Throttle Hold (THR HOLD annunciated) to guard against auto throttle runaway. It remains in this mode untila different thrust limit (other than TO) is selected.
When SPD mode is engaged, the autothrottle will attempt to control aircraft speed to the value selected in the speed window. The maximum power available in the SPD mode is determined by the current selection on the Thrust Mode Select Panel.
Automatic engagement of the autothrottle is discussed under the related pitch modes.