Need help with an Automation with MQTT


i’m a newbie in Node Red, so sorry if the question is stupid
i made an automation which ends in for example:

payload : 60

I want to use the payload to turn on a zigbee-plug for a defined time of 60 s

the payload to turn on the plug for 60 seconds is:

payload : "{"state" : "on", "on_time": 60}"

I tested this with injecting it to the mqtt-out node

How do i convert payload : 60 in payload : “{“state” : “on”, “on_time”: 60}” to send it to the mqtt-out node?