Any reason you are not using the Mios binding?
From the MQTT README:
Item myItem {mqtt="<direction>[<broker>:<topic>:<type>:<transformer>:<regex_filter>], <direction>[<broker>:<topic>:<type>:<transformation>], ..."}
This is an inbound Item so:
direction This is always “<” for inbound messages.
The <broker> is the name of your broker in mqtt.cfg
The <topic> is the name of the topic
Since you want to send a command to the switch per the MQTT README,
type Describes what the message content contains: a status update or command. Allowed values are ‘state’ or ‘command’.
so you want to use command.
To make this easier we won’t use a transformation and create a separate config for each of ON and OFF.
The <regex_filer> needs to be a regular expression that only matches for the message that message. Something like .*\"DeviceName\":\"Fornuis\".*\"Status\"\:1.
Putting it together you have
mqtt="<[mybroker:my/topic:command:ON:.*\"DeviceName\":\"Fornuis\".*\"Status\"\:1], <[mybroker:my/topic:command:OFF:.*\"DeviceName\":\"Fornuis\".*\"Status\"\:0]
There are examples all over the place, these two are the most relevant: