Welcome to Hubitat!
Presence detection basically works the same in Hubitat as it does in ST. You simply allow the user of the App to select a presence senor from an INPUT statement. Then you subscribe to its events. To be honest, there is probably no code you need to change to use any of the various Presence solutions in Hubitat. They all eventually end up being a Device that supports the Presence Capability, with the exact same attributes and events that ST uses.
This one may be slightly different, but still very simple. When you create a Pushover Device in Hubitat, it supports the Notifications Capability. So, your app needs an INPUT statement to allow the user to select the correct device. Then you simply use that device to call a command of the Pushover device to send a push notification.
For example:
input "notificationDevice", "capability.notification", multiple: false, required: false
and
notificationDevice.deviceNotification("Here is the message!")
For details of all of the Device Capabilties in Hubitat, check out the documentation
https://docs.hubitat.com/index.php?title=Driver_Capability_List