You should also read carefully the User's Guide.
The purpose of Public Sub DesignerCreateView(Base As Panel, lbl As Label, Props As Map) is explained in chapter 10.6 Display a custom view in the Designer the roles of Base, lbl and Map too.
As explained in post #5, the SelectedItem property of a Spinner is read only you cannot set it !
If you want a settable SelectedItem property in your custom view you must add it in the code as explained in post #5.
How to add properties to a CustomView is explained in detail in the User's Guide chapter 10.5 Add properties to a class, concrete examples are in chapters 10.3 and 10.4.
Any property you want for your CustomView must be added in the Class code. The original properties of any view you add in the class are not directly accessible from outsides the class.
Best regards.