Proxy Object dynamic Access

Hey Intersystems-Developer,

I have already used that and know its possible, but can't find it anymore :(

I need dynamic access on proxy objects. For example:

set key = "lastName"

set name = obj.name

set lastName = obj.key <- Not possible 

set lastName = obj.GetAt(key) <- Not possible

How can I get access to that object with my dynamic variable "key" ?

Best regards.