To test the Mollie payment API on my localhost I needed a way to receive the webhook callback on my localhost. To be able to do this I needed a service to tunnel a public URL to localhost (my local machine, as opposed to a publicly accessible URL). Here are few examples of services that provide this service:
ngrok (free plan)
These types of tools give you a publicly accessible URL that forwards the request to the localhost port, which can be used for the API callback URL while testing.