Ability to add new values using selectbox or multiselect widgets

I want to have a widget where I’m able to select from a number of options displayed in the dropdown but if the value is not in dropdown, I should be able to add a new value in there.

This is what I have noticed with the selectbox or multiselect widget. If there is a work around I don’t know about, please let me know. Otherwise, I’ll highly appreciate this feature.