I had a problem when trying to add a widget to the Scrollbox. Widget inherited from the UuserWidget class. Such a widget is well added in the designer, but in c++ code I can not find in any way how to do this. Help me, please
Ah, you mean UMG (which is technically built on top of Slate IIRC, so the calls are similar):
UScrollBox* MyScrollBox; // I'm going to assume this is created somewhere either through the UMG Designer or through a NewObject call.
MyScrollBox->AddChild( MyCustomWidget ); // Where MyCustomWidget is the widget you want to add to the scrollbox