Follow these steps to set your playbook as the default playbook:
Navigate to AI-Builder from left menu with icon
You will see list of playbooks. Scroll down till you see the playbook named "Default". In front of this playbook, you will see an event named "default" as shown below.
In Triggers, click No. You will get the question - Are you sure you want to make this the 'default" playbook" ? Select Confirm
In the option Launch this playbook on specific pages of your site, paste the URL of the site where you want to launch the playbook