That property doesn't exists in the class you are using as context. But in the rule editor, General tab, you have a section called Temporary Variables, you can create there, and reference it with @<name> in the rule. In your case, your assing will be:

assing @Namespace = Piece(AlertRequest.SourceConfigName,"|")

Here is the documentation:

Temporary Variables

User-specified temporary variables, which you can use in the rule definition. The variables must be separated by commas, for example:

FreeShippingValue,ShipMethod,PremierMember

You can reference a temporary variable in a rule definition by preceding the variable name with the @ (at sign) character, for example, @FreeShippingValue. You cannot use temporary variables outside the rule definition. To pass information to a transformation, you can instead use the RuleUserData property. For more information, see Selecting the Transformation and Target of a Send Action.