Installing Honeywell RTH9580WF on ICM280 Furnace Control Board
According to the Manufacturer's documentation, the common wire from the transformer connects to the board at pin 6 of the Molex connector.


  


According to the wiring diagram, there should be a junction off the transformer, where one wire goes to the Molex connector, and another goes to the ground (chassis).  This junction may be directly at the transformer terminals.  This would likely be the best place to connect your new C wire.


  


Make sure you use a voltmeter, to verify that those are indeed the correct pins.

If there's not an extra wire in the cable that runs up to the thermostat. Then you'll have to pull a new wire/cable.