hooking up a humidifier on a carrier infinity furnace
According to the documentation for your unit.

The HUM terminal is a 24 VAC output, energized when the blower is operating during a call for heat.
Connect an accessory 24 VAC, 0.5 amp. maximum humidifier (if used) to the ¼ in. male quick-connect HUM terminal and COM-24V screw terminal on the control board thermostat strip.
NOTE: If the humidifier has its own 24 VAC power supply, an isolation relay may be required. Connect the 24 VAC coil of the isolation relay to the HUM and COM/24V screw terminal on the control board thermostat strip. (See Fig. 32.)


So if the humidifier is less than 0.5 amperes, then you can simply use the HUM and COM terminals.  Otherwise you'll need an alternate power supply, and an isolation relay.