Place the aircraft on the runway or tarmac with 0 speed and 0 altitude and put a waypoint in the direction you want them to face. The waypoint must similarly specify 0 altitude and 0 speed and have NO actions whatsoever.
Once you give the aircraft a Takeoff action and an speed/altitude on the first WP they will position themselves at the default end and heading.
If you place them as in the first paragraph, but have some action in the first waypoint, they will start rolling and take off anyway, sometimes in amazing short field takeoffs!