Running A Macro From A Different Workbook

I have 2 workbooks (Workbook1 and Workbook2).

Workbook 1 contains a button, that calls a macro. This macro calls a macro from Workbook 2.

For this, I am using the Application.Run function.

The Macro in Workbook2 is a simple one line macro that updates cell A2 to a value of 6.

Range("A2").Value = 6

This works fine!

However, I want the the cell A2 in WORKBOOK1 to change to 6, not the cell in Workbook 2.

To add an element of difficulty, Workbook 1 could have any name (I won't know what it is, as the user could change it).

How can I get the Macro in Workbook 2 (Called from Workbook1), to update the cells in Workbook 1.