Thanks all for your suggestions.
Just to say the final word, the following code is confirmed to work on UE5.3
#if WITH_EDITOR
#include "UnrealEd.h"
#endif
...
#if WITH_EDITOR
...
UBoxComponent* BoxComponent = NewObject<UBoxComponent>(this, TEXT("NewBoxComponent"));
if (IsValid(BoxComponent))
{
AddInstanceComponent(BoxComponent);
BoxComponent->AttachToComponent(RootComponent, FAttachmentTransformRules::KeepRelativeTransform);
// deselect/select this actor to update Editor details
USelection* Selection = GEditor->GetSelectedActors();
Selection->Deselect(this);
GEditor->SelectActor(this, true, true);
}
...
#endif