You could reform the data to an object
then call the data msg.payload.set_temp
e.g.
[{"id":"474599d714e96bb9","type":"inject","z":"1d319bc3005a51a2","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"form\":{\"id\":\"0002\",\"data\":{\"data_type\":\"cmd_v3\",\"id\":\"0002\",\"n\":\"fcu_01\",\"function\":\"write\",\"item\":[{\"n\":\"power\",\"data\":\"v\"},{\"n\":\"set_temp\",\"data\":\"v\"},{\"n\":\"fan_speed\",\"data\":\"v\"},{\"n\":\"mode\",\"data\":\"v\"}]}}}","payloadType":"json","x":130,"y":280,"wires":[["4aaf250378451bf5"]]},{"id":"4aaf250378451bf5","type":"change","z":"1d319bc3005a51a2","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$$.payload.form.data.item.${$.n:$.data}","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":300,"y":280,"wires":[["3b64295684b6c8c2"]]},{"id":"3b64295684b6c8c2","type":"debug","z":"1d319bc3005a51a2","name":"debug 4 flow 3","active":true,"tosidebar":true,"console":true,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":530,"y":280,"wires":[]}]
Output
{
"power":"v",
"set_temp":"v",
"fan_speed":"v",
"mode":"v"
}