Add entry to Package Context List using C# in Package Manager

I’ll try to clarify this a bit better. When you open Package Manager, in the “Packages:” dropdown list, I’d like to add a new entry.
Currently you see: Unity Registry, My Assets, In Project, Built-In.

I found the following documentation, but this doesn’t seem to provide any C# commands on how to get or add to the list.

Any direction on this would be appreciated. Thanks!

191032-packagemanagerui-scopebutton.png

I think what I’m looking for is probably scoped packages?