Does anyone know a way to have a function tooltip pop-up for a custom made function in Excel?

For the MS written functions, i.e. SUM, AVERAGE, etc. you can have a tooltip popup with the parameters that the function takes.  But if you type in your own function you created

Ex/

=MyCustomFunction(param1, param2, ...)

the tooltip doesn't popup.  Has anyone figured how to get this done?  is it even possible?

Thanks!