One way of doing it finding a widget:
But since you trying to establish some C++ interface that you control UMG widget from C++ there better way. Widget Bluepritns are normal blurpints so they are classes based out of other class, in case of Widget Blueprints UUserWidget… and yes since they are blueprints you can make base class for it in C++, just make UUserWidget class in C++ and reparent your widget blueprint to that class or you can use blueprint creation menu (one when you create normal Blueprint) and pick you UUserWidget class, UE4 editor will detect it’s a widget class and automatically will make Widget Blueprint instead.
This have a lot of benefits, as you will be able to communicate with widget same way as you do with actors. You can make series of BlueprintImplementableEvents and variables that will let you trigger things in widget from C++