I am trying to achieve GetPackAndGo with wincom object. It always says Argument not optional

So I tried to make vba in solidwork and find that

if I do

Set swApp = Application.SldWorks
Set swModel = swApp.ActiveDoc
Set swPackAndGo = swModel.Extension.GetPackAndGo

It can get the PackAndGo Object

But if I split them:

Set swApp = Application.SldWorks
Set swModel = swApp.ActiveDoc
Set swModelExt = swModel.Extensio