Adding C wire to WiFi thermostat
Look for the junction of brown wires and attach your C wire to that

On the wiring diagram for your furnace, the brown wires are all C wires, and they come together at a junction that should be somewhere near where the mains wires come in.  Connect your C wire (which can be any unused wire in the thermostat cable) to that junction, perhaps with a length of extension wire (brown stranded #14 THHN/AWM will do the trick), and you'll be good to go.