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" 😕