Select Render WalkList Tours from the side menu and click the Edit button on the desired tour.

List Tours - click Edit button on desired tour

Go to tab Contents / Custom HTML. Scroll to the bottom of the page, you can write your HTML code here.

Contents / Custom HTML tab - HTML code editor

Click Save on top of the page.

This is an example of HTML code and the final result:

Custom HTML result displayed on top of the Tour