Derived attributes & associations are read-only, but you can also have writable associations that update other attributes or links when they are themselves set. Read here Derived settable attributes about how that works on attributes.

EAL for the Class2Selection end of the derived settable association.

self.Class2 := vInputParameter;
self.SelectionLog := self.SelectionLog + DateTime.Now.asString + ': '+ vInputParameter.Name + '\r\n'

Here is the sample model to be loaded into the designer: File:DerivedSettableExample.modlr