How can I wire the Humistat and Humidifier so that Humudifier runs only during heat cycle?
There should be a terminal on the board of the furnace supplying either 24v or 120v. Usually marked hum. That's where you pick up power for the humidifier.

For like 20 years I've been wiring the humidifier from W to C without a single issue. On a call for heat W is energized if the humidistat is calling the humidifier energizes and compleats to common.