UE UMG, when I addchild use C++, slot lost Slot properties I designed in blueprint, how can I keep it?

Question

When I designed one Slot properties in blueprint, it works just when I put it on another blueprint directly.when I dynamic create one and addchild to another widget use C++, Slot properties designed in blueprint are not works, I need to set it by code, like this:

this->Slot:SetMinimum(FVector2D.ZeroVector);
this->Slot:SetMaximum(FVector2D.UnitVector);
this->Slot:SetPosition(FVector2D.ZeroVector);
this->Slot:SetSize(FVector2D.ZeroVector);