Earlier, I posted a way to hardcode the Airplay devices in a script. But if you only have one computer, AirPort Express, or Apple TV kind of Airplay device you can forego using the name property and do something like this, which sets my lone Apple TV as the current Airplay device:
tell application "iTunes"
set current AirPlay devices to (get AirPlay devices whose kind is Apple TV)
Even though there is only a single item result from Airplay devices whose kind is Apple TV, it's returned as a list which is what current AirPlay devices takes.
The other kind values, AirPlay device and unknown, are too generic to use this way—unless you know you only have one of them.