It seems that, at least with this aircraft (B787-10), the autothrottle only goes ON if we previosly load a flight plan, which is automatically inserted in the FMC (with a lot of bugs and no-working features tough), and then engage LNAV and VNAV on the Autopilot.
It should be possible to turn ON the A/T and then select TOGA for take-off, even if the A/P is only switched ON later.