IFTTT is the way i solved this… “OK Google light” triggers a IFTTT script that does a web call to HASS setting a scene. You can use any phrase / keyword you want like “light” “dark” etc
Unfortunately this is a limitation of Alexa, there are basically no single words commands and you cannot programme them in. Using IFTTT you would still have to tell Alexa to forward the command to IFTTT with the keyword “trigger”, which is more cumbersome and longer than what you can already do: “Alexa, lights on”