First, set up the Brief Message and Detailed Message at Settings >> Labels & Messages. You can set it per language if you have multiple languages enabled. Then you enable it at Settings >> Web Booking Setup
What is a custom message and where is it displayed?
You can display a custom message on the top of the booking widget and the search result pages. It can be an important notice that you would like the bookers to know before they start the booking process. For example, you can display a notice regarding health & safety procedures that are currently in effect […]