Form auto complete number field new values

my client asked me why when he enters text into a field the value is not saved.
i explained him that this is kind of search - list field and he has to select one of the dropping items. well he wanted to automatically add new text so it will appear in the search list.
In old MS Access times there was an event triggered when a user enters a non existing value in the list box.
i figure out that the value of the textbox for numeric autocompelete is probably null or 0 until.
so how to get the entered text on leaving the field and save its textvalue?