Assuming the workbook was saved earlier
(this is a requirement, book must be saved first)

Click Insert > Name > Define
Put under "Names in workbook:":   WSN
Put in the "Refers to:" box:
=MID(CELL("Filename",INDIRECT("A1")),FIND("]",CELL("Filename",INDIRECT("A1")
))+1,32)
Click OK

The above defines WSN as a name we can use to refer to the sheetname in
formulas in any sheet. It will auto-extract the sheetname implicitly.
Technique came from a post by Harlan. For eg in any sheet, in any cell: =WSN
will return the sheetname in the cell.