Before I read your reply, I changed it to "Abre netflix en mi ordenador" and it kinda worked out. I still have to add the word "apaga" before that setence on the voice command so Alexa recognise it.
I tried your way with "Alexa, encender netflix" and now Alexa tells me that the voice command is "Alexa, activa encender netflix"
For some reason, there's always a word before my voice command that I cannot delete or change. At first it was "apaga", now is "activa" 
Hey, I've just installed the skill on the Alexa app and it's working... or kinda...
The thing is that Alexa recognizes the commands that I input on the TriggerCMD app on my desktop PC, but by default every commands has the word "shutdown" in front. (apaga in spanish since is my native language).
So for instance, I'm trying to open netflix on my PC using Alexa and I have to say: "Alexa, shutdown open Netflix in my PC".
I tried to change the commands using the Alexa app but no good, so I don't know if it is a TriggerCMD issue or configuration that I did wrong.
Thanks for the help!