To add the website booking widget, use the Booking Widget URL found within External > Websites > Manage, named Booking Widget (or create a new site) and create an IFrame (Inline Frame) on your existing website.

For those who do not know on how to make an IFrame, please refer to this link: http://www.iframecodegenerator.com/ . Add the URL that you copied in this tool. Make sure to add https:// to the src attribute!