This feature is only available on Lightning calendars. VisualForce calendars support a similar (but more limited) feature: Apply custom price calculations
Booker25 allows you to hook in to the logic behind the reservation form, giving you a lot of freedom to customize its behavior.
This article will give you a high level overview of the feature. For a deeper dive into this topic, check out these other articles:
Quick Start Guide to help you get your first implementation up and running
API Reference for details on the methods and objects you can use