determine agent for send mail step

Hi experts,

I need to determine the agent for a send mail step based on the cost center of a purchase requisition. Since we cannot call rules in the send mail step, I plan to use expressions. In my business object I would like to know how I should define this virtual attribute. I think it should be swhactor. But I do not know how to define it. Any help is appreciated.

Regards,

Gayathri