another conditional formating question

using fmp9, on a layout I have an empty field that holds a pop up menu, after a uses selects a specific menu item I need to capture their user name and date stamp the action.  Is this best done through conditional formating or a button - script    can you help with the syntax
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I would make an auto-enter field, calculated after the current user and date if the field containing the popup is not empty and prevent the field to be overwritten (do not replace existing value).
conditionnal formatting is just for field colors.
let me know if you need a quick example

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sconiAuthor Commented:
Thanks, when I see the answer it's so obvious,  I'm thankful for the help,  I don't have anyone here to talk to about this stuff.
like most of the people here I guess... this is what the forums are for!
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