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: