Create a flight plan using the default Flight Planner, and save it (use a simple name so it's easy to remember/find.
In the 777, open the CDU > RTE > SEND (LSK3L) > FLTSIM FPLS (LSK6R) > NEXT PAGE/PREV PAGE to navigate through the flight plans saved in Documents\Flight Simulator X Files, or Documents\Prepar3D v3 Files folder, and find your flightplan, select it, and it loads.
Chances are that you will need to add a DEP, and ARR, to the route so that the 777 follows the route properly.