You should be able to connect to any of the blue wires, or to the chassis where the transformer is bonded. I wouldn't use the HUM terminal, unless you've looked at the schematic and determined that it's connected directly to the transformer. If you include the model number, or a photo of the schematic. We'd be able to tell you for sure.

If you're supplying an A/C unit contactor, the furnace circuit board, and an exhaust fan from this transformer. You may find that it doesn't have the power to also supply a thermostat. There may be no problem at all, but it's something to be aware of.

For the automatic humidistat, aka what you have

Since you're using the automatic humidity controller, this gets somewhat...interesting. Wire up the R, C, W, and Gf terminals as directed to the furnace terminal block -- you will need to pull the G wire from the thermostat off the terminal block and send it to the G terminal on the auto-humidistat as well. Then, jumper the R terminal from the furnace to one H terminal on the humidity controller, and connect the humidifier valve wires to the other H and the C terminal on the auto-humidistat.

If you were using the manual humidistat

There is a yellow wire going from pin 10 of the 12 pin control board connector to the blower pressure switch i.e. the yellow wire that goes into the pin on that connector closest to the terminal block -- you'll need to tap into that wire and send it off to the humidistat as well as to the blower pressure switch. Your other wire from the humidistat goes to one terminal of the water valve, with the other terminal of the water valve getting spliced into the blue C (Common) wire from the transformer.

In general, a wire tap like that would be done by cutting the wire, crimping a female tab terminal (faston, quick-connect) on one end and a male tab terminal on the other, and then using a female/male tab terminal tap connector to make the 3-way connection. (So called "vampire tap" terminals aren't as reliable as properly made crimps.)