Need to Code on how to get previous defect status from ALM through Work flow code.

Hi,


I am new to workflow.


When Defect is rejected to QA with, Defect Status='Rejected to QA'  in ALM.


Scenario1.


When status change from 'Rejected to QA' -> 'Closed', it should enable list box called 'Reason for Invalid' field.


Please suggest which method, do i need have the code and also, need the code to get previous status validated and enable the list box field.


Would some help please :(.