When building a flightplan in the fmc page, insert the ability to put airways as a route element like the real life fmc.

entry point->airway->exit point

It's already there but it has two restrictions:

1) You need to first enter the point at which you want to enter the airway

2) You cannot have any points after the entry point in your flight plan; i.e. you need to build it sequentially, and cannot enter an airway in the middle of a flight plan.

E.G. :

NWP -> GORLO
NWP -> UL980
NWP -> Exit point of the airway

Alternativelly, you can also use airways on the routes section of this web site and beam the route down as a sequence of its points.

p.s. You need to have a Navigraph data cycle, like the default 1006 one that comes with AirTrack by default.