This tutorial will step through the process of creating a custom field in order to display "Book now" button in listing page.
Create a Custom Field Group, and ensure the type is "Listings"
Add a Custom Field to that group:
Type: Text Input
Name: Booking URL (Or what ever you want)
Description: Note: Url must start by http://
Be sure to check categories and products this should apply to, and check "Update existing orders".
Once created, take note of the Field ID that was just created, you'll need it later. (In this tutorial, the Field ID is 5.)
Open up a listing in edit view, and ensure the new created field is available.
Navigate to Plugins -> Listimia options -> Other settings (left sidebar menu)
Add the ID of the field created earlier, to Booking field ID (In this tutorial, the Field ID is 5)
Refresh your listing, and you should see something like this:
"Book now" button is displayed only if there is a value in the field.