How can I create a custom event?

In order to use a custom event that is fired when consent is given or updated, you need to create a new Window Event.
You may find this under the Implementation section, on the Data Layer & Events tab.

Click the + icon to add a new window event and specify the name of the event in the field labeled Window Event Name.

In this example we'll use "ucEvent".

To execute a script only after a user gives their consent for a specific service, you'll need to check if consent indeed was given for this service.

Hee is an example of how you could implement this:

<script>
  window.addEventListener("ucEvent", function (e) {
    if( e.detail && e.detail.event == "consent_status") {
      if (e.detail['Google Ads Remarketing'] === true) { // check for consent status of service "Google Ads Remarketing"
        console.log('Google Ads Remarketing has consent'); // add your script here
      } else {
        console.log('Google Ads Remarketing has no consent'); // add your script here
      }
    }
  });
</script>