Many of Denon devices are implemented as "Generic Serial Device" which has a very powerfull interface, but for small things like yours there is a simplified syntax.
Take a look for example at Denon DVD-A11 (just add it as a device, got Wizard->Devices->Generic Serial Devices" and click on "Ruby source Code", you can delete it later, after you see how it's done there).
You'll see on ON command something like
Which practically means send that string.
In your case you can write something like
Hope this helps.