One approach is to ignore the outer class, which represents the application as a whole, and model only the three structs as classes. You draw three classes, Booking, Customer and Room, and you draw associations from Booking to Customer and to Room. These three classes probably only have attributes, no operations (in C#, structs may have methods, but this is not recommended).
If you want to model the outer class as well, you can model the structs as nested classes in the class diagram.
More information about how to model nested classes in UML:
https://stackoverflow.com/a/57655130/5483079
In your case, you draw Booking, Customer and Room inside a compartment of the outer class (you didn't mention the name of this class). Then, you draw associations from Booking to Customer and to Room. The outer class acts a bit like a frame around an inner class diagram.
A third option is to model the structs as a composite structure inside a compartment of the outer class rectangle. But this is not a very common approach.