How to define Field of TTN uplink node?

I am going to make a flow like the following figure 3, which only select humidity or light in the TTN uplink field, but after I setting this, therer has no change in the debug window, the only change was the background of payload.humidity became more dark. However what I want is the output is only the humidity value.
How can I define the field in TTN field in correct format and how to define the topic in MQTT out to get the downlink message?( The previous topic was molly-pycom-device/devices/orangefipy/down)
Thanks very much!!!