Edges are implicit and don't really have an index of their own. So providing indices or an index range won't work. But you can make use of all kinds of selections in the selection string. "Active" will give you the active Selection, you can drag selection tags in there and put quotation marks around it (like this "MySelection") or you can use one of the selection nodes do either set or store a selection or set the active selection before you apply the chamfer.