Macro to add a value to combo box table

I have created a form in Access that contains a combo box that is attached to a table of values. I would like users to be able to permanently add values to the table that were not in the original table. Help function says that I can create a macro that will add a new value to the combo box list if I set the OnNotInList property to the name of the macro or to [EventProcedure]. I don’t know how to write a VBA macro or event procedure to do this. Can someone provide the coding for this?