You can do this in LabVIEW in a number of ways. The easiest is probably to have a text field on your front panel, and every time you change the text, you write the data to your file with the current system time. Use the event structure to trigger the write to the file. To coordinate with your data file writes, you probably want to use a single, non-reentrant VI for both data and notation writes. That way they will be automatically interleaved. The VI would take either data or text as an input, with a switch to determine what gets written. Open the file once at the beginning of your data acquisition and use the file reference as an input to your write VI.
Note that detecting when a text field changes is somewhat tricky. By default, LabVIEW signals a change in a text control when the control loses focus (you click off the text box or click the enter button on the toolbar). You can set options to make the text control send change events on every key stroke or when the enter key is pressed. You probably want to set the latter.
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