You're doing it right.
Set the first WP as Action::Takeoff
Optionally, set "Spawn parked" if you want to taxi to runway - latest RC patch puts you on runway engine/s running otherwise (use to spawn cold on runway - less than optimal)
Above works for me (atleast from shown airbase)
Try another airfield to see if you've found another bug?
You're not using a "homebase" selecting planetype and clicking "Create"?