From GTA Network Wiki
Places a player into the given vehicle and seat.
void NAPI.Player.SetPlayerIntoVehicle(Client player, Vehicle vehicle, int seat);
- player: The player to place into the vehicle. Parameter input should be in Client type.
- vehicle: The vehicle to place the player in. Parameter input should be in Vehicle type.
- seat: The seat ID. Use -1 for driver seat and 0+ for passenger seats. Parameter input should be in int type.
This will set the player into the driver seat of the hVehicle NetHandle:
NAPI.Player.SetPlayerIntoVehicle(player, hVehicle, -1);