Electrical Connection AprilAire 600M Bypass Humidifier
W1 should work

If you want the humidifier running whenever the heat's running, then using the W1 and C terminals on the thermostat connection should indeed work.

...but, I'd make sure there's a humidistat on this

However, I would make sure there's a humidistat controlling the humidifier (it can simply be in series between W1 and the humidifier, provided it's an open-on-rise/close-on-fall type) as well, as too much humidity is a rather effective way to slow-kill a house.