Copy the INS URL from the plugin and go to Paddle Developer Tools Alerts >> Webhooks page and paste the INS URL to URL Field for receiving webhook alerts. If you want to receive email alerts please enter your email address where you want to receive alerts. Scroll down and check all the webhooks checkbox under Subscriptions, One-off purchases, and Risk & disputes section.

If you are doing testing of a plugin you can use the sandbox account details as well just below real account details. Here you can enter your sandbox account Vendor ID, Auth-Code, and Public key details there.