To enter airways into the ForeFlight NavLog, go to the Maps page and open the NavLog. Select Edit to edit the route.
Before you can enter an airway you will need both an "On Ramp" (first waypoint along the airway that you are using) and an "Off Ramp" (last waypoint you are using along the airway). Enter the "On Ramp" waypoint, enter the name of the Airway, and then enter the "Off Ramp" waypoint. After entering the route information, only the name of the airway will show in Edit Route box, but every waypoint will show on the Map.
It is possible to not load every waypoint along the route and only load the "Bends" - where the route changes direction. You can change this option by going to MORE > SETTINGS > AIRWAYS DECODING > BENDS ONLY or ALL WAYPOINTS SHOWN.
Image 1. Sample entry of an airway in ForeFlight.