Which humidistat terminals should I use for my humidifier?

W-C at the thermostat terminals on your furnace's control panel is the answer to your question. This way has a few drawbacks and pitfalls, so manufacturers don't recommend it. If you know what you're doing, then you'll probably be able to get it to work that way.

If you don't have a humidifier terminal on your furnace's control panel, manufacturers recommend switching the humidifier with a current sensing relay attached to the Hot 120 volt blower fan wire that is connected to the Heat terminal. The other wires coming from the blower fan will be connected to Cool, Park, and Neutral terminals, but we don't want to use them.

The most common switching voltage for humidifiers is 24VAC, but verify what you have. If you need to switch 24VAC, then use a Model A50 Current Sensing Relay. If you need to switch 120VAC, then use a Model A51 Current Sensing Relay.

Here is a link to the Installation Instructions for Models 50 and 51 Current Sensing Relays.