Visit pushover.net to get an account and that gives you a user 'token' (long long random letters and numbers.) Then add the devices that will receive the notification.. phone/tablet, etc. Then you create an application and it responds with an app 'token' (the 2nd long long random letters and numbers.)
On Hubitat, create one (or more) virtual device(s) and assign it the Pushover driver.
Paste in the two Tokens from creating a Pushover account. Save.
Use the Pushover device(s) in Rules and Apps to send notifications.
Create an Account at http://www.pushover.net and after completing the new account process a User Key (Token) is automatically created. There's only one User Key per account and it's displayed in the upper right of the Account screen as: Your User Key.
At the bottom of the account screen, Your Applications and click on: (Create an Application/API Token) And enter a name for the Application: usually "Hubitat". Agree to the terms and click Create Application. The application key is shown as: API Token/Key. You can create other keys for other applications, but use the Hubitat one for Hubitat notifications.
On Hubitat Elevation, click Devices in the left menu and click Create Virtual Device at the top of the screen.
Device Name: Pushover
Device Network ID: psh01 <-- example: (can be anything UNIQUE)
Type: Pushover Driver
Click Save Device
In Preferences for the device you just created:
Paste the Key/Tokens in their respective fields:
user key/token in User Key.
application key/token in Application Key.
Click Save Preferences.
That's it, you're done creating -- time to start using your new Pushover notification system. Simply swap to using the Pushover device in Rules or Apps where you want to have a notification sent.