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) > Prices and Dates > Prices.

Also, if you want to add a custom form as per your requirement for Book Now button, you can add a custom form instead of a custom link by going to Admin Panel > Trips > Individual Trip(edited) > Prices and Dates > 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.