Interfaces: Forms elements, single select + add text option

I am creating a form in Interface Designer. I have fields that are single select. Unfortunately I need the person who is filling out the form to be able to do single select option, if their option is not listed then they are able to use single line of text. When I go to change the preferences it only shows that it can either be mandatory field or not. No option to allow to input text if single select option is not available.

Is there a way to make that possible or is that not feasible?