Ah so you want to find a widget blueprint. First of all i need to tell you that blueprints are classes same as C++ classes, they inherent the same way, i explain that in this video
Now widget blueprint is standard blueprint, blueprint it self is asset which contain blueprint code which is used to generate the class added to hierarchy together with C++ classes.So first get widget blueprint class is UWidgetBlueprint and oyu get it same way as that mesh and to get UClass of widget from blueprint asset you use GetBlueprintClass()
But UClass is just class identifier, you need to spawn widget:
UClass* ClassFromBlueprint = WidgetAsset->GetBlueprintClass();
UUserWidget* Widget = NewObject<UUserWidget>(this,ClassFromBlueprint);
And then you just add it to viewport with this function liek yiou do in blueprints:
There simpler way to do this, make UPROPERTY settable in blueprint with
TSubclassOf<UUserWidget*>
type, in class like AGameMode, then you create GameMode blueprint based from that class and you set that varbale, then you spawn widget based of that varable (note TSubclassOf is UClass* which limits class selection in editor). This is more nice looking solution and safe from any potential refrence issues in packageing
Also importent node, UMG is blueprint frontend for Slate (UI framework original made for editor, but it is used also in UE4 gameplay, UMG let it use it in blueprints), Slate in raw form is a lot nicer to use then UMG in C++, so if you want to define UI in C++, Slate is better: