i have done that to various bikes, but not to speedy.
in speedy it would be easy, as the battery is below the seat. just wire it straight from the battery, just put some inline fuse close to the battery(+side wire) so you don't burn up the bike.
if you want it to stop giving power when the bikes ignition is turned off, you could use rear lamps wire and some relay to connect the power only when the rear lamp is on, because it's on when the bike is on.
12volt outlet is great also for mobile phone charger during trips.