As of Unreal 5.1 implemented with a UUserWidget containing a UButton*…
void UCustomButton::SetSelected()
{
//references from UCustomButton.h (UCustomButton::UUserWidget)
//UButton* Button;
Button->GetCachedWidget()->OnMouseEnter(FGeometry(), FPointerEvent());
//Button->GetCachedWidget()->OnMouseLeave(FPointerEvent());
}