Get Username - but only once

So, I've cribbed some VBA code to pull the login name of the user into a workbook. (I readily admit that I copied this from another site, as I really have no clue when it comes to VBA.)

This code works a treat for getting the username, and I simply add the formula "=windowsusername()" into whatever cell I want to have it populate.

However, if a different user opens the worksheet, it then changes the result. (That is, if "aliceanders" opens the doc first, then saves it and send it to "bobbrewer", when Bob opens it, the user will be "bobbrewer".)

Question: how can I get this to only update the cell the first time someone opens the document?

The VBA I'm using:

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I should have explained that the worksheet I'm creating is to be used as a template for others. So, yeah...I'll be the first one opening it, and it will default to my username, then. Is there a way to save it in a 'blank' state, perhaps, and then have it update, once only, the next time it is opened?

Or, perhaps, some way of stating "the next time this particular cell is modified, lock it down"?

Shoot...I need to get in the practice of stating all my restrictions on these, too. Because the ultimate template will actually be launched from another program that gets a little finicky, I am restricted from using anything that activates upon open of the worksheet, as it appears this one does. I'm still testing whether "on activate" of a particular worksheet will work, but I doubt it will. (I've heard that "on modification" of a certain cell is possible, but I can't find the way to write that, and I'm not certain it would work in this instance anyway; I understand that requires an active modification of a cell, which this would not be.)