Step 1) you must have a HomeKit Hub. That is an Apple TV or Apple HomePod. Without this, Automations can't be created.
Step 2) In Hubitat, create a Virtual Device, choose the Virtual Presence with Switch type in the drop down and name it something useful.
Step 3) select the new device in Homekit Integration (it will be in the Switch list.)
Step 4) The remaining steps are all done on your Apple mobile device.
In Apple's Home App, at the bottom center is Automation. You will create TWO automations, one for when you arrive, one for when you leave. They just set the switch state accordingly, otherwise identical.
Step 4A) Create a new Automation by tapping the + in the upper right, Add Automation.
Tap People Arrive (or When I Arrive) Tap Next and select the Hubitat Switch you added. Make sure the state of the switch is On. That's it for this Automation. Create a nearly identical one for When I Leave (or People Leave) and select the same switch, but make sure it's selected to be Off.
That's it for connecting presence from Apple to Hubitat. Apple detects you entering the Geofence and runs the Arrive automation, which turns On the switch. Apple detects you leaving the Geofence and runs the Leave automation, which turns Off the switch. Because you selected "Virtual Presence with Switch" Hubitat will track, both as a switch and as a presence sensor, the state of Apple's presence.
Each Automation has a Test This Automation. Use them to verify in the Hubitat virtual device you created.
If you get successful results using Test this Automation, then you know that everything from your Apple Mobile device all the way to Hubitat's device is working. It all relies on Apple detecting the Geofence for your home.
I use the virtual device in a Hubitat Rule to Open my Garage Door between 7am and Sunset. Although it works reliably, there is variability in exactly when it triggers. By that I mean, when I drive up, sometimes Presence is detected 300 ft down my street, other times it detects when I get to the driveway, on rare occasions it detects presence after I've sat in the driveway, waiting for 10 seconds or so. I attribute that to where I am in Apple's queue to send a notification. If there are 300 other people entering or leaving their Geofence, I have to queue up behind them. Most of the time, I'm at the top of the queue, but not every time. 
