Alexa was fine with the skill already available. Just need to add the skill.
The next challenge was homekit/homebridge. I started trying to find an API and it's actually more complicated than I imagined. I looked out for more commercial integrations to eWelink and found the IFTTT one. That makes it "easier" with the IFTTT applet with a webhooks.
1. Create an IFTTT applet: IF webhook event name "ewe_light_on" THEN turn on the light
2. Create an IFTTT applet: IF webhook event name "ewe_light_off" THEN turn off the light
3. Create a bash shell script to call the IFTTT webhook with the webhook address.
curl -X POST https://maker.ifttt.com/trigger/ewe_light_$1/with/key/
-- The $1 variable allows input for the on/off variable.
4. Create a virtual device in Domoticz with the on commands as the script namescript:///home/pi/scripts/ewe_light.sh on
5. Add the virtual device to the Homekit Roomplan. And the device will appear in Homekit. (Recap, I've already had thehomebridge-edomoticz npm module setup.)
With Domoticz already setup, it's easy to create a virtual device pointing to the domoticz URL.