PayPal

Log in via the PayPal Developer Dashboard.
Go to My apps & credentials, set Live, and click Create App to make a new PayPal app.
Copy the ClientID & Secret Key and add them into Settings > Payments in your admin panel.
Back in the PayPal dashboard, select your newly created app (under Live).
Scroll down to Production Webhooks and click Add Webhook.
For Webhook URL, use the endpoint from Admin > Settings > Payments.
Under Event types, select All Events and click Save.