Webhook integrations can be added at Global level, in the Global processes module -> Integrations. There, click on the + button to add a new integration.
To activate the webhook, you will need to enter the corresponding URL and choose an authentication method. It can be Simple (username + password) or OAuth (Token + Client ID + Client Secret).
Once everything has been set up, click on the Integration button at the top of the screen to activate it.
Webhook integrations can also be created at Global level for Event/Contact/Organization processes specifically, and at Event level in the Workflows module -> Processes.