Index Match to data in other workbook

The easiest way to do it is to open both workbooks, and create the formula by selecting the ranges as needed, and making sure that the formula works. Then close the workbook that you want to create links to, and the references will properly update.

For example:

=INDEX('[Workbook.xlsm]Sheet'!$A:$A,MATCH(A2,'[Workbook.xlsm]Sheet'!$AC:$AC,FALSE))

Becomes

=INDEX('\\Path\[Workbook.xlsm]Sheet'!$A:$A,MATCH(A2,'\\Path\[Workbook.xlsm]Sheet'!$AC:$AC,FALSE))

That way, you don't have to worry about the proper placement of brackets and quotes....

Just be aware that the formula probably won't work well with the other book closed because it isn't a simple link and basically requires that the other workbook be opened by Excel in the background and then closed. So it may just be better to open both workbooks from a performance perspective.