WP Travel Pro plugin provides the feature of adding custom link for Book Now button. You can redirect page to any URL as per your requirement by going to Admin Panel > Trips > Individual Trip(edited) > Dates and Prices.
Also, if you want to add custom form as per your requirement for Book Now button, you can add custom form instead of custom link by going to Admin Panel > Trips > Individual Trip(edited) > Dates and Prices.
For detail assistance regarding custom booking, please refer our official documentation.
If you have any confusion then please post them in our official support forum.