This is really an Excel question rather than a OneStream one... You can probably have a simple Excel macro that does the work via a button, although I don't know if it would then work in a OneStream Spreadsheet window.
Another approach could be to have a XFSetCell call that takes that value and saves it in some dummy place, from which the dashboard can then retrieve it. To avoid issues with multiple people using the same intersection at the same time, you could keep it in a separate dummy "UserPrefs" cube, where you have a dimension representing users. It's a bit hardcore but it then allows you to store all sorts of user-specific data.
