Automatically Incrementing Invoice Number Upon Opening File Macro not working

I'll start by saying I don't have much experience with VBA and macros...

I have been trying to use a macro to automatically increment invoice numbers upon opening a file, which I found at a website (but apparently I'm not allowed to link it due to this forum's rules)

The Code is as follows:

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Something is wrong though - when I open the file, the value in D2 does not automatically increment. When I run the routine via the Debugger, it increments, but not when I open the file. This may have something to do with the code being saved as a "module" rather than a Macro, but every time I try to save it as a Macro, I either get a compile error, or else it tells me, "This action will reset your project, proceed anyway?" (note, this happens because I am trying to delete "Sub Macro1()" at the beginning of the macro).

How can I set up a proper macro that will increment the number in cell D2 every time I open the file?

Any advice would be much appreciated.

This works perfectly, thank you!

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