To setup Pushover.net notification in Proxmox VE, go to:  
`Datacenter -> Notifications -> Notification Targets -> Add -> Webhook`

Set up the following fields:
- __Endpoint Name:__ `pushover` (or whatever you want, note the character limitation)
- __Method/URL:__ `POST`, `https://api.pushover.net/1/messages.json`
- __Headers:__ Add one set of key-value pairs:
    - `Content-Type` -> `application/json`
- __Body:__: See `pushover_proxmox-notification-webhook.json` file below
- __Secrets:__ Add two sets of key-value pairs: 
    - `apptoken` -> `Pushover App Token`
    - `userkey` -> `Pushover User Key`
- __Comment:__ `Pushover.net Notification` (or whatever you want)

Adjust Pushover priority accordingly. Documentation of Pushover API can be found [here](https://pushover.net/api). 

More about Proxmox VE Webhook notifications: [here](https://pve-beta.bhgs.lan:8006/pve-docs/chapter-notifications.html#notification_targets_webhook)