Alexa Netflix commands are supported on third-generation and later Fire TVs and the Fire TV Cube.
This featured is enabled by default on all Fire TV models.
Netflix added support for in-app voice controls with the version 5.3.0 update.

If you have a Fire TV Cube, Alexa can execute such commands while your TV is turned off.
As of early 2020, it’s not possible to pick a specific episode or season of a series.
For example, say “Alexa, search for Leonardo DiCaprio movies on Netflix.”