I stumbled on the same problem, and unfortunatley, this cannot be done. You cannot have your custom widget variables have binding, like default widget properties can.
I found the answer here: https://forums.unrealengine.com/unreal-engine/feedback-for-epic/44401-bind-custom-variable-in-umg-widget
Quote:
…no blueprint variables defined in a
UserWidget can have this feature, not
without some really complex
refactoring.