I apologize in advance as I may not be much help. The addon is currently not working for me and I no longer use the Meross switches. As such, I cannot test the setup to give you a proper step by step, but I can point you in the right direction.
You need to follow the instructions for the addon on its forum thread here: Google Assistant Webserver
One of the first links on that page will send you to these instructions in order to obtain a json file from google that you need to rename to “google_assistant.json” and place in your share folder. You only need that json file, so once you have it stop following those instructions.
Start the addon, go to http://[HASSIO LOCAL IP]:9324 and follow the instructions.
Then we test broadcast to make sure everything is up and working. Enter this into a web browser:
http://[HASSIO LOCAL IP]:5000/broadcast_message?message=check check is this thing on
If it is, then we can setup your switch in HA as a command line switch.
It would look something like this:
command_on: "/usr/bin/curl -X GET http://[HASSIO LOCAL IP]:5000/command?message=turn on living room light"
command_off: "/usr/bin/curl -X GET http://[HASSIO LOCAL IP]:5000/command?message=turn off living room light"