how can I add that "+" option allowing user to custom input data for a field?

how can I add that "+" option allowing user to custom input data for a field?

as topic, when I'm checking some templates I find that it's possible to allow users to enter custom data for a dropdown field, but can't figure out where this option can be found, any idea? thanks in advance :)

in case I'm not explaining clearly, I've attached a screenshot for reference.