There's a dedicated socket on the F800 GS battery cover bracket provided for powering a Sat Nav from the Can Bus, switched via the ignition.
All you need to buy is the male plug from a Dealer or Nippy Normans
It's really simple - did it myself just last week. Remove the plastic 'tank' panel that surrounds the ignition. Underneath you'll see a black plastic dust cap on the end of a cable by the battery - remove cap and plug in. Sorted. NB if you have a Zumo 550 there's a guy selling the exact cable for this on here.