Access field data entry options

Maybe this is more complicated then I would hope, but.  I create tables and queries in Access all the time, but have never had to do more than simple data sheets. I would like to have a field on a form that could be populated via simple data entry from the keyboard with an option to fill the field with a selection from pop up-lookup list based on a predefined query. I have some predefined items, but I want the user to have the option of entering their own text item. The item they enter does not have to be in or added to the pop up-look up list.