You can find a student position number for a particular FOAP by going to the University Budgets page and clicking on 'Positions by FOAP'. You will need to have your department's fund number and organization number. Each department will have two position numbers. The "W" position number will be used for federal work study students and the "S" position number will be used for non-work study.  You can find a document that walks you through the lookup process here within the document titled "How to Report a Hire / Make a Change."