Make sure you have the service and integration already setup for Pingdom. Follow the link below on instructions of how to create integration and service
You can choose to edit or add a new one. Enter the webhook you copied in the above step.
Once you have saved it, you can use this for all monitoring. For uptime checks, navigate to the uptime section under Experience monitoring.
Select one or more checks and edit to select the webhook at the bottom of the modal.
You can choose to test the webhook but note that it will create a new incident.