Hi Keith,

>>>>I have recently installed Autocad 2020
But AutoCAD for Mac 2020 has not yet been released. Maybe you mean 2019 version?

Anyway, since 2017 version you cannot edit "predefined" commands in CUI editor. So the trick described in the discussion you referred doesn't work anymore.
The only way for now is to edit DoubleClickRoot.cui, which is inside AutoCAD application folder.